Residential Net-Zero Energy Buildings: Review and Perspective

Abstract

Advancements in residential net-zero energy buildings (NZEBs) could significantly reduce energy consumption and greenhouse gas emissions. NZEB design considerations broadly categorize into energy infrastructure connections, renewable energy sources, and energy-efficiency measures. There is a lack of systematic literature review focused on recent progress in residential NZEBs. Therefore, this work provides an overview of each category including recent developments (last ≈ 10 years), aiming to provide references and support of wider and more successful implementation of residential NZEBs throughout the globe. The discussed energy infrastructure connections include electrical grids, district heating/cooling networks, and energy storage options including vehicle-to-home and hydrogen storage. Renewable energy sources considered here are solar photovoltaic and solar thermal, wind, and biomass including micro combined heat and power (CHP) systems. The final category detailed is energy-efficiency measures, which include improved building envelope designs, efficient HVAC systems, efficient domestic hot water systems, and phase change material integration. Within these categories there are many technology options, which makes selecting the ‘best’ configuration more difficult but allows design flexibility to adapt to local climates and other considerations (i.e. building codes, energy resources, costs). This paper provides references and highlights technology options to achieve residential NZEBs throughout the world.

1. Introduction

Buildings consume 30 % to 40 % of the yearly primary energy in developed countries, and approximately 15 % to 25 % in developing countries [1]. In the United States, buildings account for around 40 % of primary energy consumption, and therefore 40 % of the total U.S. CO₂ emissions and 7.4 % of the total global CO₂ emissions [2]. More narrowly, residential buildings comprise 21 % of the U.S. energy consumption, and are therefore responsible for about 20 % of the total U.S. CO₂ emissions [2]. Therefore, reducing energy use in homes would substantially lower energy use and emission of greenhouse gases (GHGs) [3].

There is increasing world-wide interest in net-zero energy buildings (NZEBs) to reduce emissions. In this paper NZEBs are defined as buildings that generate at least as much energy as they consume on an annual basis when tracked at the building site [4]. The United Kingdom was the 1^st country to mandate NZEBs on a large scale, with the goal of producing zero-carbon homes by 2016 [5]. The European Union parliament has introduced a directive regulating that all new buildings constructed starting January 2021 should be “nearly zero-energy” buildings [6]. France has set ambitious targets for energy-positive houses by 2020 [7]. The U.S. Department of Energy (DOE) has targeted “marketable zero-energy homes in 2020 and commercial zero energy buildings in 2025” [8]. California will require all new residences to be net-zero by 2020, and all commercial buildings by 2030 [9]. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) set a goal of market-viable NZEBs by 2030 [10]. Many other countries have also set long-term goals to implement NZEBs [11].

There are many approaches to realize residential NZEBs, either through minimized building energy demand (via improved building designs and/or occupant behaviors) or increasing renewable energy generation. There is a lack of systematic literature review focused on recent progress in residential NZEBs. A systematic review and professional perspective can greatly contribute to broader and better implementation of residential NZEBs towards a sustainable future. Therefore, this paper gives an overview of the public literature covering the methods and recent developments (≈ last 10 years) in residential NZEBs across the world. Although there have been other review papers on NEZBs, this review provides various technology options in a systematic way (energy infrastructure connections, renewable energy sources, and energy efficiency measures), focusing on residential buildings. The research is conducted following the flowchart presented in Fig. A1. Section 2 discusses NZEB development trends in terms of worldwide research and projects, and broad design considerations. Section 3 highlights the design category of energy infrastructure connections. Section 4 covers the design category of renewable energy sources. Section 5 reviews the design category of energy-efficiency measures. Section 6 summarizes the design options and presents conclusions. The goal of this paper is to provide references and to highlight the technology options to achieve residential NZEBs throughout the world.

2. Development and methods of NZEBs

This section presents the development trends of NZEBs in terms of worldwide research and demonstration projects. An ambitious but reasonable target is to lower the average energy consumption in buildings to passive house levels (i.e. “passivehaus”, limiting the heating load intensity to 15 kWh/m² and the total primary energy demand to 60 kWh/m² [12]). This effort would require a large turnover of existing buildings; a recent study (Fig. 1) showed for selected, relatively cool countries, that at least 60 % to 80 % of existing houses in selected countries would need to be converted into NZEBs to meet the target. Current requirements for new construction will achieve 10 % to 80 % NZEBs, varying by country [13], showing there is room to grow the NZEB market and achieve more energy-sustainable building stock. Although Fig. 1 is mainly for cold areas, it is strong evidence that NZEBs will be an essential pathway towards a sustainable future.

Fig. 1.

Reduction of average energy intensity with implementation of residential NZEBs in selected countries [13].

2.1. Research trends and demonstration projects

Fig. 2. shows the increasing number of publications on NZEBs within the past 23 years, based on the Web of Science database [14]. Although the publications don’t cover all the research work, they strongly reflect the past years’ dramatic increase of efforts in NZEBs. This development trend is likely to continue under various countries’ sustainability goals and consumer demand trends.

Fig. 2.

Variation of the number of publications on NZEBs in the past years.

Demonstration projects have also been widely presented. Fig. 3 shows the distribution of various types of demonstration NZEBs around the world [15], [16]. The demonstration buildings include: 47 special typology buildings (e.g. hotel, hospital, sports hall), 37 education buildings, 20 settlements (groups of buildings, including row houses), 164 small residential buildings, 73 office buildings, 40 apartment buildings, and 31 “others.” Residential NZEBs are the majority, and are mainly located in Europe and North America [16].

Fig. 3.

Distribution of different types of NZEBs in different countries [16].

2.2. Broad design considerations

There are multiple variations of “net-zero” goals including [17]–[19]: “net-zero energy, zero energy costs, zero energy emissions, nearly net-zero energy, zero emission, zero carbon, net-zero exergy”. Torcellini et al. [20] provide four different common definitions:

1. Net-zero site energy: the building generates at least as much energy as it consumes, per year, when tracked at the site;

2. Net-zero source energy: the building generates at least as much primary energy as it consumes, per year, when tracked at the source;

3. Net-zero energy costs: the utility company pays the building owner at least as much money as the owner pays the utility company during the year;

4. Net-zero energy emissions: the building exports at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.”

Building designers are concerned with on-site energy to comply with energy codes; building users are focused on energy costs; governments are concentrated on source energy supply; while environmental protection organizations are concerned with energy emissions.

Regardless of which NZEB definition is used, a set of three common principles and features generally apply:

• NZEBs connect to energy infrastructures;

• NZEBs exhibit a significantly lower energy demand via energy-efficient measures;

• NZEBs have energy generation from renewable energy sources.

Many design principles and rules have been recommended around these features. For example, Fanney and Healy [21] summarized ten principles for designing residential NZEBs:

1. Design for comfort and function;

2. Establish an airtight building enclosure;

3. Provide controlled ventilation;

4. Incorporate insulation that exceeds present energy code requirements;

5. Ensure the building enclosure controls water and moisture movement;

6. Orient the building to maximize renewable energy production;

7. Select efficient mechanical equipment;

8. Select efficient lighting, plumbing fixtures, and appliances;

9. Use energy modeling to predict total energy use, size on-site renewables, and identify high-value improvements to energy efficiency;

10. Develop project plans that coordinate and commission systems.

This paper groups the research and methods to achieve NZEBs into three broad categories: energy infrastructure connections, renewable energy sources, and energy-efficiency measures (Fig. 4).

Fig. 4.

Methods to achieve residential NZEBs.

The electrical grid is the most common energy infrastructure connection and is convenient for importing and exporting electricity. Gas pipe networks are another important energy infrastructure but generally can only be used for energy import and must be combined with other energy infrastructure for renewable energy export. Synthetic natural gas (SNG) could be exported to the grid from renewable sources, but the process is complex [22] and requires significant capital investment and technical expertise to produce and install products that meet grid quality standards. It is likely that SNG production will be done at the utility scale, rather than for individual homes.

District heating and cooling systems can also accommodate bidirectional energy flows as excess thermal energy can be returned to the hot or cold streams. For example, excess heat from solar thermal collectors or the excess cold produced by solar cooling systems can be exported. Metering thermal energy is challenging because the associated flow and temperature difference measurements can be expensive. Biomass/biofuel can be either obtained on-site or from a distribution network (typically an off-site renewable energy source); biomass is often converted to usable energy off site, and this energy is subsequently delivered for end use via the electrical grid or district heating and cooling systems. Energy storage can boost self-consumption of renewable energy by storing during times of excess and discharging during renewable energy deficiency, providing monetary benefit to building owners when energy import cost is higher than the export rebate. Energy storage can further be used to generate income by leveraging changes in energy prices; power is purchased during times of low demand and price and exported to the grid when the energy demand and market price are high.

Renewable energy sources include solar, wind, and biomass, all of which can be harvested locally or offsite, where offsite resources may be purchased from a utility. The energies can be used in diverse ways as they can all be converted to electricity, heat, or both (combined heat and power, CHP).

Energy-efficiency measures include building envelope design, as well as efficient heating, ventilation, and air-conditioning (HVAC) equipment, appliances, controls, occupant behaviors, etc.

NZEB designs should consider available infrastructure connections, available energy sources, climate conditions, and economic factors. Achieving NZEBs requires high energy efficiency to reduce loads, and then implementation of renewable energy sources to balance the energy use. Torcellini et al. [20] developed a hierarchy of NZEB options in order of preferred application, Fig. 5. Building energy efficiency measures (Option 0) are the priority [23] since savings last the lifetime of the building and don’t have conversion or transmission losses associated with renewable energy sources. The renewable generation options, in order of preference are:

Fig. 5.

Hierarchy of energy savings and renewable production options to achieve NZEBs. Option 0 is combined with one or more of Options 1–4 [20].

(Option 1) Generation on the building footprint (which minimizes land use);

(Option 2) On-site generation from on-site renewables;

(Option 3) On-site generation from off-site renewables; and

(Option 4) Off-site supply.

This hierarchy encourages harvesting renewable resources within the building footprint and at the site, to avoid transmission losses. It is likely that on-site generation is also more attractive and exciting to the building owner as they can visualize their investment and energy savings.

The Torcellini et al. [20] hierarchy is useful for design strategy from a broad perspective but requires refinement for making lower-level design decisions. For example, how much insulation is enough? Are solar photovoltaic panels more cost effective at saving energy than some energy efficiency measures? Answering these questions requires detailed analysis. Kneifel et. al [24] simulated the energy use of a home considering multiple options for: insulation thicknesses, window specifications, air tightness, heat pump efficiency, water heating, and photovoltaic (PV) array size. They simulated every combination of all options (i.e. a full factorial), for a total 240 000 designs. They found design options that achieved net-zero energy at lower life cycle cost (LCC) than a Maryland, U.S., code-compliant house. The operating energy was reduced by the PV (61 %), reduced air leakage (21 %), more-efficient HVAC equipment including both heat pump and a heat-recovery ventilator (HRV) (15 %), and more-efficient domestic hot water (DHW) equipment (9 %). However, all the energy savings and generation measures increased the embodied energy of the building including: 26 % for installing the PV system, 7 % for air-leakage reduction, 1.4 % for higher-efficiency HVAC equipment, and 4 % for more-efficient DHW production [25]. On net, the ratio of increased embodied energy to reduced lifetime operation energy ranged from 1:1 to 1:89 (i.e. the saved operation energy was 89 times greater than the increased embodied energy), with the highest ratio coming from using a HPWH rather than a conventional electric water heater. Despite the increased embodied energy in the NZEB, the total life cycle energy reduced to 625177 kWh, compared to 1694000 kWh for the code-compliant house (a 63 % reduction). Interestingly, the PV system had better cost performance than some of the energy efficiency measures, but that included a 30 % PV tax credit. However, PV will likely be more cost effective than some energy efficiency measures regardless of the tax credit, given the recent downward trend in PV price [26], [27].

The progress and development of NZEBs within each of the design categories (Fig. 4) are reviewed in the following three sections.

3. Energy infrastructures

NZEBs require energy infrastructure to manage short-term imbalances in renewable supply and building demand. Excess renewable energy should be exported, and energy import is required when renewable resources are insufficient to meet the building load. Depending on the energy infrastructure connections, the NZEBs are classified as on-grid or off-grid. On-grid NZEBs [28] are connected to a utility such as an electrical grid, district heating/cooling system, or a biomass/biofuel distribution network [17]. Off-grid (i.e. ‘autonomous’, ‘standalone’ or ‘self-sufficient’) NZEBs [28], [29] only use on-site renewable energy generation and significant energy storage to meet the building demand, which is generally more difficult and expensive to implement than on-grid systems. This section describes the energy infrastructure options for both on- and off-grid NZEBs.

3.1. Electrical grid

Electricity is required by almost every building, so NZEBs generally require means to import or generate electricity. Meeting the electrical demand entirely through renewable generation is difficult because of timing mismatches in renewable resource supply and building demand (i.e. winter nights with heating demand but no solar resource). The electrical grid fills the gaps in supply during times of low renewable resource and can receive power when the generation exceeds the building demand.

Excess electricity doesn’t necessarily need to be exported to the electrical grid, and instead could be stored in batteries or converted to thermal energy for a district heating/cooling network. However, energy storage is currently relatively expensive, and district energy networks are only available in limited areas and require additional investment for construction. Moreover, there is unavoidable energy loss during the conversion process. The electrical grid is usually the best option and is the most widely used energy infrastructure for NZEBs, owing to the convenient connection and no additional cost for energy storage devices or other energy infrastructure.

3.2. District heating

District heating systems are widely used in regions with high heating loads and concentrated communities, due to the high energy efficiency of centralized energy conversion and management. District heating systems can incorporate renewable heating technologies like solar-thermal collectors, wind heat generators (direct conversion of wind energy into heat, e.g., Joule machine) [30], heat pumps, biomass boilers, and biomass CHP generators. Like the electrical grid, district heating systems can import/export energy as needed to accommodate deficient/excess renewable generation.

Lund et al. [31] concluded that district heating is vital to reaching a 100 % renewable heating and cooling supply for Denmark. They defined the concept of smart thermal grids as “a network of pipes connecting the buildings in a neighborhood, town center or whole city”, served from centralized plants and distributed heating/cooling systems, which can both import and export thermal energy. Centralized plants foster the use of CHP systems including those using biomass; the heat is applied directly, and the power can drive heat pumps. Further, the pipe network can recover industrial waste heat for use in homes. Lastly, district heating systems can readily integrate large-scale solar-thermal collectors.

Around half of buildings in Denmark are integrated into district heating systems, with 77 % of the heat generated at CHP plants [32]. Nielsen and Möller [33] investigated NZEBs connected with district heating systems to assess where single-family NZEBs should be built and how the total heat generation in district heating systems is affected by the excess heat produced from the NZEBs’ solar thermal collectors. They found the excess heat from NZEBs benefited district heating systems by decreasing combustible fuels. In some cases, seasonal heat storage was necessary to utilize all the renewable production during summer months.

Noris et al. [34] analyzed different European NZEBs to assess how different primary-energy and carbon-emission weighting factors (relating site energy use to source energy/emissions) would influence the choice of energy systems, including ground-source heat pumps (GSHPs), gas condensing boilers, biomass boilers, gas CHP systems, or district heating. NZEBs are most readily implemented in Europe using biomass boilers, since the primary-energy weighting factor is low, and the remaining energy can be harvested using PVs installed within the existing roof area. Gas boilers have high weighting factors, so it is more difficult to achieve NZEBs using the existing roof area. The authors point out that the use of heat pumps and district heating would be promoted by strategically lowering the weighting factors for electrical and district heating grids; the lower weighting factors would reflect national targets of increased use of renewable energy electricity in these grids.

3.3. Energy storage

Energy can be stored in the form of heat, cold, potential energy, chemical energy (e.g., batteries), etc. when there is excess renewable generation, and discharged when there is deficient renewable generation. Short-term storage can be used to meet demand a few hours or days later, while seasonal storage can be discharged months later. Energy storage can increase utilization of the renewable energy at the building, reducing the import/export of energy, yielding financial benefit to the building owner when imported energy is more expensive than the rebate for exported energy. The energy storage also enables the building owner to participate in the balancing of the energy market; energy is purchased and stored when the grid has excess capacity and the price is low (e.g. during times of high solar energy production), and the energy is sold back to the grid when the demand and price are higher. Building owners need to decide if the benefit of a storage system is worth the increased initial cost and system complexity. Off-grid autonomous NZEBs require significant energy storage systems.

Excess electricity can be stored in batteries, converted into thermal energy (through a heat pump or electrical resistance heater) and then stored using sensible or phase-change energy storage techniques [35], or converted into chemical energy (e.g. producing hydrogen and oxygen from water using electrolysis [36]). Excess heat can be directly stored as thermal energy, converted into electricity (though an Organic Rankine Cycle, ORC [37]) and then stored in batteries, or converted into chemical energy (e.g. absorption or adsorption energy storage [38], [39]).

All the energy storage systems and the associated energy conversion equipment make NZEBs more complex and require additional investment. Nevertheless, for remote locations without grid connections, off-grid NZEBs could be a good option.

Stahl et al. [40] and Voss et al. [41] presented an off-grid NZEB in Freiburg, Germany (Fig. 6). The grouping of state-of-the-art energy-saving technologies with high-efficiency solar systems minimized the imbalance between the solar irradiation and the energy demand in winter. The remaining imbalance was managed using a seasonal energy storage system comprised of a water electrolyzer driven by the excess photovoltaic electricity generated in summer. The hydrogen and oxygen produced using electrolysis was pressurized and stored, and then reconverted to electricity by a fuel cell. The hydrogen was also catalytically combusted for cooking and space heating. The total annual energy use of <10 kWh/m² was met exclusively with solar energy. Their data, collected over three years, showed that it was possible to construct homes with nearly-zero heat demand in the Central European climate. The occupants gave positive feedback about their experience, which stimulated further work for off-grid residences.

Fig. 6.

A Schematic of an off-grid NZEB showing the energy supply system [40], [41].

Platell and Dudzik [42] investigated an off-grid NZEB using an ORC for CHP. They proposed a short-term “steam buffer”, which offered high-temperature heat storage using a porous ceramic, metal foam, or microchannel material. Hot CO₂ vapor from the CHP cycle charged the steam buffer; the heat then dissipated into the storage material and stored as sensible heat. The steam buffer was an efficient regenerative heat exchanger with high energy densities, reaching approximately 150 Wh/kg at a 600 °C storage temperature. In addition, the power density could be as much as 10 kW/kg, so a relatively small amount of material can be used to store intermittent renewable energy and provide large amounts of power.

On-grid NZEBs with partial storage are much closer to being cost effective than off-grid NZEBs. Partial energy storage is useful to the building owner and the utility for load shifting and better utilization of on-site renewable generation but does not require the significant investment to achieve a fully off-grid NZEB. Vehicle-to-home (V2H) technologies use idle electric vehicle (EV) battery power as a storage system to buffer electric power from renewable sources, and to help supply emergency backup power. Electric vehicles also have the potential for shifting or regulating the load and peak power profiles of the building system by charging during off-peak hours and discharging during peak hours [43], [44]. For this kind of energy infrastructure connection, the electrical grid is still the primary means to manage the energy while the vehicle battery storage plays an auxiliary role. As a downside to V2H systems, the vehicle battery lifetime is reduced by the extra dis/charging cycles; [45] estimated that the nominal 10.6 year lifetime is reduced by 0.4 years to 2 years with 1 h to 8 h of daily V2H use. Vehicle batteries reach “end-of-life” when the batteries still have 75 % of their original capacities, so the batteries can be repurposed to less-demanding stationary energy storage (e.g. electrical energy storage in a home).

Alirezaei et al. [46] investigated the role of V2H technology to manage electricity in a NZEB. A V2H system was coupled with the solar photovoltaic sources as a supplement to power from the grid in Orlando, Florida. The PV electric power was stored in a primary stationary battery, while the EV received electricity from the stationary battery and/or from the grid during off-peak hours and supplied electricity during on-peak hours. This system reduced grid-imported electricity for some months and generated income from exported surplus electricity in other months, offsetting the capital costs of the PV panels and energy-storage technologies.

Munkhammar et al. [47], [48] investigated the integration of an EV to buildings with PV to increase the self-consumption of PV power for NZEBs in Sweden and the United Kingdom. Maximizing the self-consumption of PV electricity lowered costs as self-produced power was cheaper than the grid power. However, the ratio of self-consumed to total annual energy actually decreased from 31.6 % to 25.5 % with the EV. This is because the EV increased the total consumed energy by 1.53 MWh (37 %), but the self-consumed power only increased 0.13 MWh (10 %). The self-consumed power benefit was relatively small because the EV was often either not at the home or was already at full charge when PV power was available.

Fuel cell vehicles convert fuels (e.g., hydrogen) to electricity and heat with zero pollutant emissions, and have been integrated with some residential buildings [49], [50]. Cao et al. [51] investigated the integration of a hydrogen fuel cell vehicle with a 150 m² residential NZEB in Helsinki, Finland. The surplus PV or wind generation drove an electrolysis process to create hydrogen, which was later discharged to provide power and heat. They found that a 14 kW wind turbine or a 178 m² PV could nearly meet the net-zero goal of the building and the vehicle. They also compared NZEBs integrated with a hydrogen vehicle or an electric vehicle. The NZEB with the hydrogen vehicle required a 4 kW larger wind turbine or a 35.6 m² larger PV, because the hydrogen-vehicle integrated system was less efficient than the electric-vehicle integrated system [52]. In a follow-up study, they conducted a technical and economic analysis for the hybrid system with considerations of hydrogen vehicle refueling methods. They concluded that the external refueling station was techno-economically superior to the on-site hydrogen refueling system, due to the high cost of the small-scale on-site hydrogen system (the small-scale electrolyzer was particularly expensive) [53].

4. Renewable energy sources

Renewable energy sources include solar, wind, and biomass. Solar energy is the most accessible, followed by wind and biomass [54]. However, solar energy is only available in the daytime on sunny days, while wind and biomass energy are available both day and night in wind-rich or biomass-rich regions. Solar or wind energy can usually be harvested on site but needs off-site sources when the installation space is insufficient. Wind turbine location must comply with setback regulations designed to minimize impact of noise and “shadow flicker” (caused by turbine blade passing between the observer and the sun) [55]. Biomass/biofuel can be either obtained on-site or imported from distribution network; the network could include vehicle delivery of raw biomass or converted energy via electrical grids and district heating/cooling infrastructure (more feasible for urban buildings). All the renewable sources can be converted into thermal or electrical energy. In the Unites States, renewable energy comprised about 7 % of consumption for residences in 2019 [2], where the sources were 5 % geothermal, 31 % solar, and 65 % biomass (wood and wood-derived fuels). Considering the small fraction of renewable energy use there is significant room for growth.

On-site solar PV systems are currently the dominant renewable energy technology [56] since they: have relatively low (and reducing) cost [26], [27], the marginal cost is relatively independent of size, and can be readily installed on or integrated with building roofs and facades. [57]. However, other on/off-site renewable energy technologies have also been considered to achieve NZEBs. Marszal et al. [57] used LCC analysis to ascertain the optimal levels of energy efficiency and renewable energy generation, including on- and off-site options. The two on-site options included: (1) photovoltaic, (2) micro combined heat and power. The three off-site options were: (1) off-site windmill, (2) share of a windmill farm and (3) purchase from an electrical grid sourced with 100 % renewable energy. They showed for the on-site generation options, energy efficiency should be the priority (rather than renewable generation) to design a cost-optimal NZEB. For the off-site options it is more economical to invest in renewable energy systems (rather than energy efficiency).

4.1. Solar energy

Solar energy can be harvested in many ways (Fig. 7), including PV, solar thermal, and combined PV and thermal (PV/T). Depending on installation location PV systems are categorized as either off-site or on-site. On-site PV is usually applied when the installation space within the site area is sufficient and has unobstructed access to sunlight; otherwise, off-site PV should be considered. Building-applied PV (BAPV) is a subset of on-site PV where the modules are installed on the exterior of finished roofs or walls. Building-integrated PV (BIPV) is another subset of on-site PV that entails using PV modules as exterior building features instead of conventional construction materials, replacing the outer surfaces of roofs, façades, balconies, and walls. BIPV technology is popular and commercialized, with a projected market of $7 billion by 2024 [58], [59]. (Note all currency figures are in $USD). Besides electricity generation, BIPV can also reduce the building loads for space cooling or heating through the shading effect [58], [60]. Lawrence Berkeley National Laboratory (LBNL) tracks the installed cost for PV systems (Fig. 8) in each state of the U.S [26], [27]. The costs in 2016–2017 varied from $2.90/W to $5.00/W, with an average of $3.70/W in 2017. EnergySage reported a lower average U.S. price for 2018 of $3.05/W [61].

Fig. 7.

Various solar energy technologies for NZEBs.

Fig. 8.

Installed cost of residential PV systems in the U.S. [26], [27].

Solar energy is also used for heating water and air. Solar water heaters are used for domestic hot water, hydronic heating, or with absorption or adsorption cooling systems. Qerimi et al. [62] simulated applying solar thermal collectors citywide to buildings in Kosovo and reported a solar energy fraction of 51 % to 70 % and a payback of 9.6 years for residential buildings. Excess heat generation during summer can be stored in a seasonal thermal energy storage system and discharged in winter. The solar air heater can be used for space heating, ventilation air preheating, or even clothes drying.

PVs typically convert 10 % to 20 % of the insolation to electricity, with remainder either reflecting or dissipating. PV/T systems harvest the waste heat into water or air streams [63], which provides thermal energy and improves PV efficiency by allowing the collectors to operate cooler. Further, PV/T systems save space because they don’t require additional area to collect thermal energy. PV/T systems can be uncovered and covered [64], where covered systems include a sheet of glass in front of PV panel with an air gap in between. Uncovered systems operate cooler and produce equal or more electricity than PV-alone systems but provide lower temperature thermal resource than traditional solar thermal collectors. Covered PV/T systems produce temperatures similar to solar thermal collectors but produce less electricity than the PV (because of the lower PV efficiency at higher operating temperatures). An indirect use of hot water from a PV/T system is to circulate it through a ground heat exchanger to reduce ground thermal imbalance for a GSHP system.

Various types of solar energy systems can be combined with each other and with other forms of renewable energy to maximize generation where installation space is limited. Fong and Lee [65] investigated a NZEB with renewable energy technology installed on all available exterior surfaces (Fig. 9) in the subtropical regions of Hong Kong where high cooling loads can make it difficult to achieve net-zero using PV alone. The roof and walls were covered with PV panels, and a flat-plate solar collector was placed on the roof. In addition, four small wind turbines were installed at the four corners of the roof. They found that the residential NZEB could be realized with nominal PV efficiencies of 13 %, and energy-conscious occupant behaviors (e.g. reasonable increase in summer setpoint, and switching off air-conditioner, lighting, or equipment when not being used).

Fig. 9.

A NZEB using various PV systems and small wind turbines [65].

Solar system selection should include evaluation of actual product efficiencies, solar sources, building load profiles, installation spaces and costs. Good et al. [66] studied three alternative solar energy systems for a residential NZEB in Norway, including: (1) solar thermal collector and PV, (2) PV/T, and (3) PV only (with the efficiencies listed in Table 1). Their calculations showed better efficiency for PV and heat pumps than with solar thermal collectors, so the best choice for renewable energy equipment was to use PV only. They also compared uncovered and covered PV/T, where the uncovered system produced as much electricity as the PV-only system, and had an average summer thermal output temperature of 40 °C. The covered system produced less electricity but had an average summer thermal output temperature equal to that of the stand-alone thermal collectors, 66 °C. The authors commented that this study was based on commercially-available solar products, where the larger market and number of options for stand-alone PV and solar thermal collectors likely disadvantaged the PV/T systems.

Table 1.

Efficiencies of ST (solar thermal), PV, and PV/T modules [66]

Module	PV type	Total area (m2)	Electric efficiency at STC* (%)	Rated electric power (Wp)	Thermal zero-loss efficiency η0 (%)
STavg	Flat plate	2.00	–	–	80
SThigh	Flat plate	2.00	–	–	85
PVavg	Poly-Si	1.65	15.8	260	–
PVhigh	Mono-Si	1.64	20.3	333	–
PV/Ta	Mono-Si, uncovered, uninsulated	1.64	17.4	285	61.4
PV/Tb	Poly-Si, covered, insulated	2.26	12.0	240	71.5

^*STC: standard test condition

Solar thermal energy can be converted to electricity using Rankine power cycles where the working fluid is either steam (i.e. Solar Rankine Cycles) or low-boiling-temperature refrigerants (i.e. Organic Rankine Cycles, ORCs). Hassoun and Dincer [67] investigated a NZEB using a cogeneration ORC system energized by water heated in solar collectors and cooled by water from a solar-driven absorption chiller. The ORC used an ammonia solution that boiled at low temperatures, had a nominal turbine capacity of 15 kW, and was connected to battery banks. The outputs of the cogeneration system included electrical power, cooled air, and hot water, and the inputs included electricity for pumps and solar thermal energy. They computed the energy and exergy efficiencies of all these energy flows in the subsystems and overall system, where the overall system energy efficiency varied from 37 % to 60 % depending on the time of day. Small-scale ORCs with acceptable efficiencies are still under development; similar to air conditioning, the working fluids of ORCs should also be carefully selected by considering efficiency, safety, cost, environmental impact, etc.

To achieve autonomous NZEBs, Platell and Dudzik [42] suggested using the small-scale solar-powered CHPs. They expressed the possibility of downsizing the solar-thermal power system by integrating parabolic trough concentrators into the building envelope. The steam engine was powered by solar energy when available and by other locally available fuels when the sun was absent. The surplus heat produced by the CHP was stored in the ground for use during winter. Another option they discussed was the CO₂ transcritical power cycle (Fig. 10(a)), which can avoid the two-phase flow instability of the steam generator. The efficiency was on the same order of magnitude as most other micropower systems (Fig. 10(b)).

Fig. 10.

Thermodynamic process and electric efficiency of the transcritical CO₂ power cycle [42].

4.2. Wind energy

Wind is one of the oldest sources of renewable energy and has had substantial growth in recent decades [68]. Table 2 lists the classification of horizontal-axis wind turbines based on rotor diameter and power rating [69]. For on-site NZEBs, it is more reasonable to use small-scale wind turbines, with rotor diameters ranging from 3 m to 10 m and power capacities of 1.4 kW to 20 kW. However, small-scale wind turbines have much higher marginal costs than medium- and large-scale wind turbines. The Small Wind Certification Council [70] has certified seven wind turbines for micro- to large-scale commercial applications (Table 3 [71]). The total cost per rated annual energy output is about $5/kWh for the larger wind turbines ranging from 5.2 kW to 10.4 kW, $7/kWh to $9/kWh for smaller wind turbines ranging from 1.5 kW to 2.5 kW. Therefore, larger wind turbines are more cost-effective, and they can be applied to larger homes or shared within housing developments and multifamily buildings.

Table 2.

Classification of horizontal-axis wind turbines based on rotor diameter and power [69]

Scale	Rotor diameter (m)		Swept area (m2)		Standard power rating (kW)
Micro	0.5	1.25	0.2	1.2	0.004	0.25
Mini	1.25	3	1.2	7.1	0.25	1.4
Small household	3	10	7	79	1.4	16
Small commercial	10	20	79	314	25	100
Medium commercial	20	50	314	1963	100	1000
Large commercial	50	100	1963	7854	1000	3000

Table 3.

Small wind turbines available in the USA [70], [71]

Turbine	AWEA rated annual energy (kWh)*	AWEA rated power at 11 m/s (kW)	Peak power	Total cost ($)	Total cost per annual energy ($/kWh)**
#1	13 800 kWh	8.9	12.6 kW @ 16.5 m/s	65, 000	4.7
#2	16 700 kWh	10.4	11.3 kW @ 12.0 m/s	83, 000	5.0
#3	8950 kWh	5.2	6.1 kW @ 17.0 m/s	45,000	5.0
#4	9920 kWh	5.5	6.7 kW @ 16.0 m/s	55, 000	5.5
#5	3420 kWh	2.1	2.4 kW @ 14.0 m/s	23, 800	7.0
#6	2420 kWh	1.5	1.7 kW @ 13.5 m/s	22, 350	9.2
#7	3930 kWh	2.5	3.0 kW @19.5 m/s	–

^*SWCC Certification Type: AWEA 9.1–2009

^**Annual Average Cost = Total Cost/AWEA Rated Annual Energy

Wind turbines can be attractive for buildings in dense cities with limited area on the roofs and façades for solar panels. Fong and Lee [65] combined small wind turbines at roof corners with PV solar collector on the roof and BIPV surrounding external walls, aiming at achieving net-zero by making the most of building on-site space in subtropical regions like Hong Kong (Fig. 9).

Compared to PV and solar thermal collectors, wind turbines are less popular due to the higher initial cost and limited locations with high and consistent levels of wind. But it may become a competitive option in wind-rich regions if the costs reduce. Iqbal [72] presented a feasibility study of a wind-turbine-based NZEB in Newfoundland, Canada, where PV panels perform poorly due to low solar resource. Wind turbines were attractive because the average annual wind speed is relatively high, 6.7 m/s, and the population density is low so there are many locations for unblocked access to the wind. A 10 kW wind turbine could meet all energy requirements of an energy-efficient single-family home with an area of 158 m².

Elkinton et al. [73] investigated wind-powered NZEBs in five U.S. cities. They compared the energy performance and economics for various house sizes, seven turbine models, and several heating systems. They concluded that wind-powered NZEBs were generally more expensive (except in the warmest climate zone) than NZEBs heated by natural gas. The best wind-powered designs in the U.S. cities of Great Falls, MT, Goodland, KS, and Oklahoma City, OK cost slightly more, about $1000 (considering construction and operating costs) over a 20-year lifetime. They also found that large-scale wind turbines applied to a community of homes were less expensive on a per-kW basis. The most favorable results were in the U.S. cities of Kahului, HI and Amarillo, TX, where wind-turbine-based NZEBs were less expensive than natural gas-heated homes respectively by $11 000 and $4600 (over the 20-year lifetime). In those locations the electricity rates were high (Kahului $0.235/kWh, Amarillo $0.128/kWh, compared about $0.083/kWh for the other cities) so the renewable generation provided significant benefit. Further, these locations received little-to-no benefit of using the low-cost natural gas heating, since the heating loads were low.

Wind turbines have a dependence on variable wind speed while PV systems have a dependence on variable sunshine. To overcome reliance on a just one renewable source and to prevent oversizing renewable components, the use of hybrid PV/wind systems has been advocated in other publications [74], [75]. Cao et al. [76] compared the hybrid PV/wind/hydrogen vehicle/fuel cell system (Fig. 11) in the Finland and Germany. They parametrically varied the fraction of PV to combined PV/wind capacity and found the optimal values of 20 % for Finland and 60 % for Germany, which reflected the ratio of locally-available renewable sources. However, because the kW-scale hydrogen system is relatively expensive, this hybrid zero-energy system is not cost effective for the near future.

Fig. 11.

The hybrid PV/wind/hydrogen vehicle/fuel cell system [76].

4.3. Biomass energy

Biomass energy is typically used to drive co-generation systems, including CHP [77] and combined cooling, heating, and power (CCHP) systems [78]. These systems more fully utilize the embodied fuel energy by recovering waste heat, realizing total efficiencies as high as 90 %. Micro-CHP systems typically have electrical outputs in the range 0 to 2 kWe and are therefore suitable for NZEBs at residential scales [79]. Fig. 12 provides a classification of the primary available CHP types and applications [80]. Commercially-available micro-CHPs incorporate a Stirling engine, ORC, internal combustion engine, or fuel cell. CHP systems with external combustion can accept a wider variety of fuels since the combustion process is isolated from the working fluid of the power cycle by a heat exchanger; this flexibility is attractive because the systems can more readily use the variety of forms of renewable biomass fuels.

Fig. 12.

Main CHP technologies and application fields [80].

Biomass-based CHP systems are attractive to use during times of low insolation or in dense cities where buildings are close together and shade each other. Cao et al. [81] summarized different kinds of commercial micro-CHP products, with the technical specifications listed in Table 4. The electrical-power-to-thermal (P/H) output ratio is the smallest for ORCs (0.06 to 0.15) and Stirling engines (0.08 to 0.26), followed by internal combustion engines (0.30 to 0.40), and is the largest for fuel cells (0.50 to 0.59). The CHP type should be selected to match the electrical-to-thermal demand profile of the building to avoid wasting usable energy. Mohamed et al. [82] investigated which biomass micro-CHP systems’ electrical-to-thermal ratio best matched the load profile of a Finnish single-family NZEB. They assessed two control strategies for each CHP, (1) thermal tracking (CHP controlled to meet thermal load) and (2) electrical tracking (CHP controlled to meet electrical load). With thermal-tracking control, CHP systems fueled by bio-syngas (a gaseous fuel produced by reacting renewable-hydrocarbon feedstock with steam, carbon dioxide, or oxygen) achieved net-zero primary energy using a CHP with capacity larger than 2 kWe and a P/H ratio greater than 0.5, while CHPs fueled by natural gas never achieved net-zero energy. Net-zero energy was more readily achieved with bio-syngas because it is produced from renewable sources. With electrical-tracking control, bio-syngas-fueled CHPs achieved net-zero energy with low P/H ratios, below 0.2, whereas natural-gas-fueled CHPs never achieved net-zero energy but had significant energy reductions for P/H ratios 0.2 and greater. For both tracking strategies, there was a best P/H ratio for each CHP size that yielded the minimum net primary energy consumption. Micro fuel cells are very expensive at present and have a relatively short lifespan of (10 to 15) years, and the stack needs to be replaced about every 5 years [83] compared to 25-year lifetime for PVs (typically value estimated, using the mean 0.8 %/yr degradation and a “failure” limit of 20 % total degradation [84]). A domestic SOFC (solid oxide fuel cell) CHP system currently costs about $20,000 to $30,000 per kWe [83]. There are major efforts to reduce the cost; the U.S. Department of Energy aimed for system cost of $1500 by 2020 for a 5 kW natural gas-fueled fuel-cell CHP system [85], while Staffell and Green [86] suggested a long-term cost target of $3000 to $5000 for a 1 kW to 2 kW system by 2020.

Table 4.

Technical specifications of representative commercially-available micro-CHP products [81]

Micro-CHP type	Nominal electrical power (kWe)	Nominal thermal power (kWth)	Electrical to thermal ratio	Fuel
Stirling engine	0.8	5.5	0.15	Automotive grade diesel
	1.0	7.5 to 12.0	0.08 to 0.13	Natural gas
	1.0	6.4	0.16	Solar heat, biogas, and natural gas
	1.0	6.0	0.17	Natural gas, other fuels (biomass, solar and waste heat) under instigation
	1.2	5.0	0.24	Hydrocarbon, fossil fuel, biomass
	1.4	5.4	0.26	Pellets (wood)
	2.0 to 9.0	8.0 to 25.0	0.08 to 0.25	Bio-gas, natural gas
ORC	1.0	6.8 to 18.0	0.06 to 0.15	Natural gas, liquefied petroleum gas, etc.
	1.0	8.8	0.11	Solar heat
Internal combustion engine	1.0	3.3	0.30	Natural gas, liquid propane
	1.0	2.5	0.40	Natural gas, liquid propane
Fuel cell	0.3	0.6	0.50	Natural gas
	1.0	2.0	0.50	Natural gas
	1.0	1.8	0.56	Natural gas
	1.0	1.7	0.59	Natural gas

Mohamed et al. [87] analyzed seven biomass CHP systems, including a standalone wood-pellet Stirling engine micro-CHP, five shared woodchip micro-scale and small-scale CHPs (direct-combustion Stirling engine, updraft-gasifier Stirling engine, indirect fired gas turbine, internal combustion engine coupled with a gasifier, direct combustion ORC), and a domestic-scale hydrogen fuel cell. The results showed that a domestic-scale biomass CHP wasn’t optimal for NZEBs, while a locally shared biomass CHP was superior due to its higher overall efficiency and electrical-to-thermal ratio.

Marszal et al. [57] compared three types of micro-CHP systems, a biogas fuel cell, a hydrogen fuel cell, and a biomass Stirling CHP, with solar and wind energy resources. Their theoretical analysis found that the biomass Stirling CHP was the most economical on-site renewable energy system, though the systems are not yet ready for application to real buildings since the reliability should be improved and the CHP is still a developing technology at a building scale. A major disadvantage of the micro biogas and hydrogen CHPs is the high excess electricity production.

Thiers and Peuportier [88] conducted life cycle analysis for NZEBs using a wood pellet micro-CHP and a wood-pellet condensing boiler. With both options the NZEBs had low energy consumption and good environmental performance, with total equivalent emissions (including those related to construction, operation, and demolition) ranging (10 to 24) kg CO₂ eq/(m²·yr), far below the mean level in France (32 kg CO₂ eq/(m²·yr) in 2007). The global warming impact of the building operation was actually negative for some cases because of the use of renewable wood for heat and power production; however, the global warming emissions related to construction & demolition made the total global warming impact positive.

Biogas can be produced from wastes, residues, and energy crops, and will act as an important renewable energy source in the future. Anaerobic digestion has been regarded as one of the most energy-efficient and environmentally beneficial technologies for bioenergy production [89]. In NZEBs, kitchen food waste could be a good raw material for on-site bioenergy production. Apart from on-site production, the biomass can also be transported from outside sources. Besides being used for cogeneration systems, biomass can also be used to generate heating or cooling without electricity generation, using biomass boilers, absorption or adsorption chillers, or gas-driven heat pumps [90]. These systems must be combined with other renewable energy systems, like PV and wind turbine, to generate electricity.

5. Energy-efficiency measures

Energy-efficiency measures for NZEBs are broadly grouped into two categories: (1) reducing the building load and (2) more efficiently meeting the load. Examples of load reduction include improved building designs (envelope, layout, orientation, etc.), solar shading, and efficient occupant behaviors (opening windows during favorable outdoor conditions, switching off HVAC when not in use, DHW draw profile to match HPWH production capacity, etc.). The load can be met with less energy input through selection of efficient mechanical systems (e.g. HVAC and DHW), equipment and controls, and efficient building appliances (lighting, refrigerator, washing machines, dryers, etc.).

The surveyed literature showed that practitioners focused on the energy-efficiency measures of (1) efficient mechanical systems and (2) improved building envelope. Fig. 13 shows the variables that a surveyed group of 28 experts selected for optimization during the design of NZEBs [91]. The experts most often optimized systems (54 %) and controls (54 %) followed by envelope (50 %). Systems and controls were considered the most complex and dynamic design parameters, and so design optimization was critical. Other energy-efficiency measures like layout & geometry (25 %), internal gains (18 %), occupancy (11 %), and location & climate (7 %) were less-often optimized, perhaps because these parameters are decided earlier in the design process before energy designers are involved. Note that occupancy and location & climate are typically not parameters that can be optimized for residential buildings. As for the optimization criteria in NZEB design, the experts selected one or more objective functions in the following percentages: energy (100 %), cost (64 %), comfort (36 %), carbon emissions (18 %), lighting (7 %), and indoor air quality (4 %).

Fig. 13.

Percentage of experts using listed optimization (a) design variables and (b) objective functions for designing NZEBs [91].

5.1. Improved building envelope designs

Building thermal loads can be lowered by using enhanced thermal insulation, enhanced thermal capacitance, increased airtightness measures, optimized orientation/shape, optimized window-to-wall ratios, enhanced window glazing, solar shading, passive solar technologies, etc. [92], [93]. Increasing the insulation thickness and use of “continuous insulation” to minimize thermal bridging is recommended, especially in heating dominated climates [94]. Windows should have an R-value of at least 0.88 m²∙K/W (higher in heating-dominated climates) and be air and water tight [21]. Window area and construction should be carefully considered to minimize annual energy use; windows have large thermal losses relative to walls, but they can also be used to passively harvest solar energy for heating in winter. Window shading can minimize solar gains in summer when the sun is higher in the sky and allow larger solar gains in winter when the sun is lower, particularly for south-facing windows (in the northern hemisphere) [21]. It is important to avoid overheating the house when using designs that incorporate significant passive solar collection [95].

Building airtightness is typically characterized by a blower door test that measures the leakage rate (often expressed in terms of air changes per hour), under a 50 Pa pressure differential between the inside and outside air. The NIST (National Institute of Standards and Technology) NZEB achieved an airtightness of 0.63 h⁻¹, by “wrapping an air-barrier membrane completely and continuously around the exterior sheathing of the roof and walls”, as well as “providing appropriate air sealing to the foundation and at the windows, doors and all wall/roof penetrations” [21], [88].

Moran et al. [96] analyzed the life cycle cost and environmental impacts (in terms of life-cycle energy use and global warming potential) of NZEBs in the temperate oceanic climate of Ireland. They concluded it was best to focus on minimizing the space heating loads through highly-insulated and airtight envelopes, rather than installing less insulation and a large renewable energy system.

Harkouss et al. [97] optimized multiple criteria for NZEBs using a simulation of the building located in diverse climatic zones of Lebanon and France, aiming to improve NZEB design and facilitate decision making at the preliminary design stages. Their investigated design parameters included external wall and roof insulation thickness, window glazing type, window-to-wall ratio, and HVAC set points. After optimizations, the annual thermal loads decreased by 7 % to 33 %, and the life-cycle cost (LCC) decreased by 1 % to 31 % in different climates.

Energy consumption decreases as insulation increases for heating-dominated buildings. However, in cooling-dominated buildings with significant internal heat loads, there is a level of insulation, called an “inflection point”, beyond which adding insulation actually increases the cooling load since the building doesn’t dissipate much heat when the outdoor temperature is cooler, i.e. nighttime or cooler days [98]. Similarly, overdesigned airtightness may also increase the cooling energy use in summer. Gupta and Kapsali considered whether NZEBs designed to lower heat loss by increasing airtightness measures and increasing insulation thickness have an increased chance of overheating and insufficient ventilation [99].

5.2. Efficient HVAC systems

Efficient HVAC systems are described in terms of options for ventilation, dehumidification, and heat pumps.

(1). ventilation options

NZEBs are usually constructed to have low air infiltration, and therefore require mechanical ventilation to remove indoor contaminants and provide “fresh” air. This is often accomplished by exchanging indoor air for outdoor air. Heat-recovery ventilators (HRV) and energy-recovery ventilators (ERV) can be used to transfer sensible and latent (ERV only) heat from the exhaust air to the outdoor air, to minimize the thermal load introduced by the outdoor air. Ng and Payne [100] analyzed the impact that an HRV had on air-source heat pump (ASHP) energy use, for a residential NZEB in Maryland, U.S. The HRV reduced HVAC energy by up to 36 % in winter, but could actually increase HVAC energy by up to 5 % increase in mild seasons. In a following study, Ng et al. [101] evaluated how ventilation affected the indoor air quality and energy use in the same NZEB. They found that lower outdoor air ventilation rates reduced energy consumption but elevated indoor contaminant levels, while higher outdoor air ventilation rates could cause a failure to meet the net-zero energy target.

Wu et al. [102] compared HRV and ERV ventilation systems against mechanical ventilation without recovery, in terms of energy, economic, and comfort. The HRV and ERV respectively lowered the annual HVAC energy by 13.5 % and 17.4 % and lowered the annual building energy by 7.5 % and 9.7 %. They also found that the ventilation options all yielded similar thermal comfort, since the heat pump was able to handle the relatively small change in load. Under an “instantaneous markup” policy, where imported electricity was billed at a retail rate of $0.153/kWh and exported electricity was credited at the wholesale rate of $0.0866/kWh, the HRV and ERV respectively had simple paybacks of 15.4 years and 9.0 years.

HRVs and ERVs significantly reduce the energy use the coldest winter months and hottest summer months (up to 20 % of monthly ASHP energy) owing to the large outdoor-indoor temperature differences [103]. ERVs save more energy than HRVs in the cooling season of humid climate zones because they reduce the latent ventilation load. Neither recovery ventilator provides significant benefit in mild “shoulder” seasons (i.e., spring/fall) or in mild climates. In these situations, the recovery ventilators can actually increase electricity use since they provide little benefit but use additional energy to operate a 2^nd fan (only one fan is needed for ventilation without recovery).

Passive ventilation options (i.e. natural ventilation), driven by wind or buoyancy, have also been considered for NZEBs [104], [105]. Grigoropoulos et al. [106] compared natural and mechanical ventilation for Eastern Mediterranean homes. They found that natural ventilation reduced the energy demand by up to 20 % but caused more uneven temperature distribution throughout the house. Ellis et al. [107] simulated a NZEB passively cooled using only natural ventilation. With improved architectural designs and strategies the net-zero goal was achieved with good thermal comfort, without mechanical cooling, even during the warmer months.

A novel ventilation technique is to directly exchange heat between the air and the ground; these earth-to-air heat exchangers can pre-condition outdoor air (Fig. 14(a)), or re-condition indoor air (Fig. 14 (b))[108], [109], for both heating and cooling. For both systems, the preheating effect from the ground reduces the heating load in winter, while the precooling effect reduces the cooling load in summer.

Fig. 14.

Earth-to-air heat exchangers connected to buildings [110].

(2). dehumidification options

For humid climate zones, dehumidification is essential and consumes large amounts of energy. Air-conditioning equipment with dedicated dehumidification (DD) can supply dry, temperature-neutral air to living space when the humidity is high but cooling is not required; in these systems hot refrigerant vapor exiting the compressor bypasses the condenser and warms the air exiting the evaporator before the air is delivered to the house [111]. The dehumidification heat is shuttled to the outdoor condenser so DD systems operate relatively efficiently, compared to portable or “whole-house” (i.e. single-packaged) dehumidifiers that reject the heat indoors (and therefore increase the cooling load) [112]. Further, heat pumps with DD use the larger condenser integrated with the outdoor unit, rather than a small one packaged with a “whole-house” dehumidifier. With larger condenser surface area DD systems can reject the heat at lower temperature and pressure, and subsequently with greater cycle efficiency.

Wu et al. [102] compared three dehumidification systems for a NZEB: (1) a single-packaged separate whole-house dehumidifier, (2) an ASHP with no DD (“ASHP only”), and (3) an ASHP with DD. They found that “the ASHP with DD lowered the annual HVAC energy by 7.3 %, lowered the annual building energy consumption by 3.9 %, and lowered the initial cost by $3293 compared to the separate dehumidifier option. The ASHP-only option was the least expensive, but the thermal comfort was the worst: 1649 h with the percentage of people dissatisfied (PPD) > 10% and 2463 h with relative humidity above 50 % (compared to the DD option with 809 h PPD > 10% and 584 h with relative humidity above 50 %)”.

Another advanced dehumidification system recommended for residential NZEBs is desiccant dehumidification [111], [113]. Fang et al. [114] compared various dehumidification schemes, concluding that the air conditioner with a desiccant wheel dehumidifier outperformed the air conditioner with the DD mode (described in the preceding two paragraphs). Kozubal et al. [115] proposed a compact desiccant-enhanced evaporative air conditioner, combining the liquid-desiccant and evaporative-cooling technologies (Fig. 15).

Fig. 15.

Desiccant enhanced evaporative air conditioner, combining the liquid desiccant and evaporative cooling technologies [115].

(3). heat pump options

Heat pumps are commonly used for residences. ASHPs have simpler configurations and lower cost, while GSHPs typically have higher energy efficiencies [116]. Mohamed et al. [87] evaluated a Finnish residential NZEB “based on four criteria: primary energy, site energy, equivalent CO₂ emission, and energy cost”. They compared five heating options including “electric heating, district heating, a GSHP, a light-oil boiler, and a wood-pellet boiler”. Compared to the conventional heating options, GSHPs reduced the size of PV systems needed to realize NZEBs. Doroudchi et al. [117] assessed a NZEB with an EV connected to the building’s energy system, where heating was supplied either by a GHSP or a district system. The GSHP reduced the annual cost by up to 11 %, compared to the reference building without vehicle-to-home mode. Further, the GSHP coupled with a PV array reduced the annual energy imported by 5 % and exported by 60 %.

The high initial cost of GSHP systems remains a barrier to adoption. Norton and Christensen [118] studied “four heating options (solar heating, GSHP, natural gas furnace, and electric resistance heating) for a residential NZEB in a cold climate (Denver, U.S.)”, and found the lowest-cost option was the natural gas furnace. Wu et al. [102] compared GSHPs with varied borehole size to an ASHP for a NZEB in Gaithersburg, U.S. They found that “the GSHP reduced the annual HVAC energy by 26 % for 2 boreholes and by 29 % for 3 boreholes and reduced the annual building energy consumption by 13 % for 2 boreholes and 15 % for 3 boreholes”. The GSHP required a smaller PV array to reach net-zero energy than the ASHP, but the overall cost (PV + HVAC) was still lower using the ASHP. They also conducted simulations for NZEBs across the U.S. and found that GSHPs save energy in cold climates, saved little energy in mild climates, and sometimes actually used more energy than ASHPs in hot climates [103]. Note that the underground thermal balance should be considered when designing GSHP systems, especially in the extremely cold or hot regions; enlarged borehole fields or hybrid GSHP systems may be good option to maintain long-term efficient operation. The cost-effectiveness of GSHPs could be improved by optimal design and advanced GHX types. There is a trade-off between energy efficiency and GHX cost, as the GHX length (and cost) increases, the energy efficiency increases but eventually reaches a point of diminished marginal savings. In addition, the GHX geometry (vertical or horizontal; U-pipe or spiral) and group layout (for apartments requiring a group of GHX) also have a great influence on the GSHP system efficiency [119].

Exhaust ASHPs [120] use the exhaust air as a heat source/sink and can be applied in residential NZEBs. Different from an HRV or ERV, an exhaust ASHP can achieve an exhaust air temperature lower than the outdoor air temperature in winter and higher than the outdoor air temperature in summer, and therefore recover more energy.

To enhance the ASHP efficiency in very cold or hot climates, it is promising to use advanced ASHPs, including those with two-stages [121], cascade cycles [122], or vapor injection (Fig. 16(a)) [123]. These advanced ASHPs perform well in extreme conditions but may perform worse than basic ASHPs in mild conditions [124]. As a result, it is important to switch between the basic and advanced cycles depending on the ambient temperatures [125].

Fig. 16.

Advanced ASHP systems.

Solar-assisted ASHPs extract heat both from the ambient air and from solar collectors [126] and are an energy-efficient option for residential NZEBs. Marszal and Heiselberg [127] assessed the LCC of a Danish net-zero home with different HVAC systems “including a solar-assisted heat pump, a GSHP, and a district heating grid”. They concluded the solar-assisted heat pump used the least amount of energy. Ran et al. simulated an “integrated solar air source heat pump” (Fig. 16 (b)) that used a flat-plate solar collector as the outdoor evaporator instead of a fan-coil unit [128]. When the evaporator temperature was higher than the demand temperature, a thermosyphon circulated the refrigerant to move heat without electricity input to the compressor.

5.3. Efficient DHW systems

Water heating also comprises a significant share of energy use in residences, so efficient DHW systems are vital to realizing NZEBs. Conventional water heaters incorporate electric heating elements, gas burners, or solar collectors. Electric and gas water heaters produce relatively low-temperature hot water using high-grade energy, yielding low primary energy efficiencies. More-efficient options include heat pumps with desuperheaters [129] and heat pump water heaters (HPWHs) [130]. Solar water heaters are also very efficient but require large installation areas and the heating capacity is affected by weather conditions.

Noguchi et al. [131] studied a NZEB in Canada using the heat pump desuperheater for water heating, and found the system reduced water-heating energy consumption by approximately 700 kWh/yr (21 %) for a hot water load of 150 L/day at 55 °C. Wu et al. [102] investigated a NZEB with a solar water heater and a HPWH. The water was first heated by a solar subsystem, and then by a HPWH, if needed, before being distributed to the plumbing fixtures. For the same NZEB, Balke et al. [132] used simulations to compare three water-heating methods including an electric-resistance water heater, a HPWH, and a HPWH with solar-thermal preheating, with respective annual COPs of 0.95, 1.90, and 2.87.

Biaou and Bernier [133] simulated different DHW systems for NZEBs in Canada equipped with PV modules for on-site electricity generation. Four alternatives (Fig. 17) were considered: a conventional electric heater, a GSHP desuperheater with auxiliary electric elements, thermal solar collectors with auxiliary electric elements, and a HPWH indirectly connected to a space-conditioning GSHP. They considered the solar collectors as the best solution since the building could then use the least peak PV power, 8 kW. In a follow-up study, the same authors presented a simple economic analysis to determine the sizes of solar-thermal and -electric arrays that best achieved a net-zero energy hot water system [134]. In Los Angeles (U.S.), the optimal system combined 4.5 m² of solar-thermal collectors with a 2.06 m² PV array, which provided a simple payback of 11 years. For Montréal, the optimal system combined 12 m² of solar-thermal collectors with a 5.2 m² PV array, and had a simple payback of 29 years.

Fig. 17.

Different water heater systems investigated in residential NZEBs [133].

Solar-assisted ASHPs are also an energy-efficient water heating option that harvest both solar energy and energy from the outdoor air [135]. In addition, the CO₂ transcritical HPWH is a highly-efficient and compact alternative for DHW in residential NZEBs [136] and CO₂ is a low-GWP refrigerant (GWP=1). The CO₂ HPWH could outperform the HPWHs using other refrigerants due to the good match of temperature glides on the refrigerant side and water side.

5.4. Phase change material (PCM) integration

Thermal energy storage technologies can help solve the problem of mismatched timing and capacity of energy supplied by renewables and building demand. Unlike the high-grade energy storage introduced in Section 3.3, PCMs are commonly used for low-temperature thermal energy storage (e.g., −20 °C to 200 °C) [137]. In summer, PCMs can store the excess cold produced by heat pump under cool conditions for later cold supply under hot ambient conditions [138]. PCMs can also store and release energy for heating. Use of PCMs provides dual benefits for the HVAC system; the heat pump capacity can be smaller since the PCM can add to the HVAC capacity, and the PCM can be charged under more favorable outdoor air temperatures (lower temperature lift gives a better Carnot cycle efficiency).

Fiorentini et al. [139] described a novel solar-assisted HVAC system consisting of an air-based PV/T and a PCM unit in a ducted system for a retrofitted NZEB. The system provided benefit year-round by harvesting daytime solar radiation during winter, and radiating heat to the sky at night during summer. The PV/T provided heating or cooling (radiative cooling in summer night) directly to the indoor space or to charge the PCM. The PCM was later used to condition the indoor space or pre-condition the outdoor air.

Kedzierski et al. [140] used a transient model to investigate two configurations of PCM for residential cooling in a NZEB. The system with the integrated-PCM evaporator showed no benefit compared to a conventional system with a larger evaporator and a smaller compressor, while the system with a remotely-stored PCM tank exhibited significant energy savings, between 6 % and 33 % depending on the PCM thermal resistance. Note that PCMs with higher thermal conductivity need to be developed to enhance heat transfer and system capacity.

5.5. Miscellaneous efficient options

There are many energy efficiency options that don’t group into building envelope design, HVAC, or DHW categories. “Smart” technologies, including smart electric meters and smart controls, can help achieve efficient and comfortable NZEBs [141]. Occupants can select energy-efficient indoor setpoints (i.e. lower in winter, higher in summer) and load schedules to help achieve the NZEB goal [142]. Using energy-efficient lighting and major appliances (washing machines, dryers, refrigerators, etc.) has a dual benefit of directly reducing electricity use and reducing the cooling load on the HVAC equipment [143].

6. Discussion and conclusions

The main technologies and features of different NZEB approaches discussed here are summarized in Table 5, including energy infrastructure, renewable energy, and energy-efficiency measures. Each of these categories is briefly discussed in the Section 6.1 through Section 6.3.

Table 5.

Summary of various NZEB approaches

Energy infrastructure connections
NZEB approaches	Technologies	Remarks
Electrical grid	• PV, PV/T, BIPV, solar CHP (i.e. ORC) • Wind turbine • Biomass CHP	• The most widely used, with easy accessibility
District heating	• Solar hot water • Wind heat generator • Biomass boiler, biomass CHP	• Requires district heating networks • Smart thermal grids can accept exported thermal energy
Energy storage	• Electrical energy storage (battery, electric vehicle) • Thermal energy storage (sensible, latent) • Chemical energy storage (hydrogen, hydrogen vehicle, fuel cell vehicle)	• Full energy storage can realize off-grid net-zero goals for remote buildings or facilities • Vehicle battery storage increases local use of renewable energy and reduces energy import/export.
Renewable energy sources
NZEB approaches	Technologies	Remarks
Solar energy	• Solar hot air, solar hot water • PV, PV/T, BIPV, solar CHP	• The efficiency and cost are different for different system • Unit cost relatively independent of installation size
Wind energy	• Wind turbine	• Small-scale wind turbines are generally less cost effective than medium- and large-models • Hybrid PV/wind systems can be used to overcome the dependency on a single renewable source and to avoid the oversizing of renewable components
Biomass energy	• Micro-CHP, including Stirling engine, ORC, internal combustion engine, and fuel cell	• The electrical to thermal ratio and cost differ amongst micro-CHPs • The micro fuel cells are very expensive at present and have a relatively short lifetime
Energy-efficiency measures
NZEB approaches	Technologies	Remarks
Improved building designs	• Increased thermal insulation • Increased thermal capacitance • Higher levels of airtightness • Optimized orientation/shape • Window-to-wall ratio • Window glazing, solar shading • Passive solar technologies, etc.	• Increased insulation and airtightness are effective in heating-dominated buildings but may increase energy use for cooling-dominated buildings
Efficient HVAC systems	• Ventilation (HRV, ERV, natural ventilation, earth-to-air heat exchanger) • Dehumidification (dedicated dehumidification mode of heat pump, desiccant dehumidification) • Heat pump (ASHP, exhaust-air ASHP, low-temperature ASHP, solar-assisted ASHP, GSHP, advanced GHX types)	• HVAC system must be selected to match the local climate
Efficient DHW systems	• Solar water heater • Desuperheater of air-conditioner • Heat pump water heater • Solar-assisted ASHP	• DHW system must be selected to match the local climate
PCM integration	• PCM cold storage • PCM heat storage	• PCM thermal resistance will greatly affect the energy benefit
Miscellaneous	• Smart control • Efficient occupant behavior • Efficient lighting • Efficient appliance	• Some technologies (e.g., efficient lighting, efficient appliance) reduce electricity use directly and also reduce the HVAC load

6.1. Energy infrastructure

The electrical grid is the most widely adopted and accessible energy infrastructure for NZEBs since most houses are already connected. District heating can be applied to NZEBs for space heating or water heating and can accept exported thermal energy. Vehicle-to-home technology is an emerging electrical energy storage method to partially bridge mismatches in demand and supply. Excess electrical energy can also be used to create stored hydrogen, which is later discharged as electricity and/or heat. However, it increases the complexity of the energy system and incurs losses during energy conversion.

6.2. Renewable energy

In recent years, solar energy has been the dominant renewable energy source for residential NZEBs, largely because of the easy availability, reducing cost, and unit cost relatively independent of installation size. There are many options for solar energy harvesting to meet the needs of a building; options include solar thermal collectors (both air and water), PV, PV/T, BIPV, BAPV, and solar CHPs. PV/T systems have the potential to improve the overall solar utilization and efficiency, which can reduce installation area. BIPV technology is promising in dense cities where the buildings have minimal roof area for PV installation. The extra shading provided by BIPV needs to be considered since it influences the cooling and heating loads.

Wind energy is generally available and practical in fewer locations than solar energy but has the benefit of being available during the night and on cloudy days. A wind turbine could supplement a solar energy system when there is deficient installation area, or to reduce reliance on a single energy source. However, wind turbines are expensive for residential NZEBs, because small-scale wind turbines are generally less cost-effective than medium- and large-scale wind turbines.

Biomass energy is independent of weather and therefore attractive when the resource is readily available. Micro-CHPs harness the energy in biomass to make useful heat and power, where CHP options include Stirling engines, ORCs, internal combustion engines, and fuel cells. Fuel cells have the largest electrical-to-thermal ratio (0.50 to 0.59), which best matches the demand ratio of most homes. Micro fuel cells are very expensive at present but could be cost effective in the future with the aggressive development.

6.3. Energy-efficiency measures

Climate affects the applicability of some energy-efficiency measures. Increased insulation and airtightness tend to create greater savings in heating-dominated buildings. These efforts are less effective for cooling-dominated buildings and can even be counterproductive if the insulation prevents natural cooling during prolonged times of lower outdoor temperature. An annual building simulation that considers the local climate is required to select the cost-optimal level of insulation.

There are many advanced options for efficient ventilation systems. HRVs and ERVs have been commonly used but are not always beneficial. When the temperature and enthalpy differences between indoor and outdoor are small, ventilation heat recovery provides little benefit but use more electricity to operate the supply and exhaust fans. Smart control of HRVs and ERVs could optimize their use based on indoor and outdoor temperature and humidity. Other novel ventilation systems such as natural ventilation or earth-to-air heat exchangers can be considered.

Efficient dehumidification options include using dedicated dehumidification reheat coils for heat pumps, or a desiccant system which can use renewable energy and waste heat. Relatively inefficient systems use electric reheat, or “whole-house” packaged dehumidifiers.

Heat pumps are readily integrated with NZEBs. ASHPs have simple configurations, low maintenance, and low cost, which make them attractive for residential applications. Exhaust ASHPs extract heat from the exhaust air, realizing more heat recovery compared to an HRV or ERV. Low-temperature ASHPs with high efficiency should be developed to operate in very cold regions, to compete with the operating cost and primary fuel use of fossil-fuel systems. Solar-assisted ASHPs combine energy from the ambient air and solar thermal collectors for efficient heating. GSHP systems have also been used in residential NZEBs due to their higher efficiency, especially for heating in cold regions. Advanced GHX types like spiral coils and energy piles can further reduce energy consumption by GSHPs. The high cost of GSHP systems remains a major obstacle to wider adoption.

Energy-efficient DHW systems include solar water heaters, air conditioners with desuperheaters, HPWHs, and solar-assisted ASHPs. The best option depends on the climate features and load profiles. The CO₂ transcritical HPWH, featuring high energy efficiency and high compactness, is suitable for residential NZEBs but is currently very expensive.

For both HVAC and DHW technologies, more-efficient systems tend to me more costly and complex. Technologies that produce significant energy and cost savings for the particular climate zone should be prioritized.

PCM integration with HVAC or DHW can help manage the mismatched timing of building demand and renewable energy supply. Development of PCMs with higher thermal conductivity would improve the dis/charging efficiency by lowering the heat transfer resistance and required temperature lift.

Smart controls, efficient lighting, efficient appliance, etc. also contribute to NZEBs by reducing the building energy demand. Moreover, efficient lighting and efficient appliances can also lower the HVAC cooling load. The technology described in this paper can only make the building net-zero “ready”, so the occupants must have reasonably energy-efficient behaviors to actually achieve net-zero operation.

There is a lack of systematic literature review focused on recent progress in residential NZEBs. The intention of this literature survey was to highlight available options for achieving a NZEB, though no single “best” configuration was presented. The summary and perspective contribute to provide references and support of broader and better implementation of residential NZEBs throughout the world. There is no single NZEB configuration that is optimal for all climates, regulations, building codes, and markets; the designer will ultimately need to select the suite of technology and building parameters to adapt to local conditions and specific requirements. While the large number of options increases the design cost, it also increases the flexibility in the design. When considering options, it is important to consider component interactions, such as a reduction in required heat pump capacity enabled by use of recovery ventilators. Annual performance simulations are recommended to compare various designs. The limitation of this work is the focus on technology advancement and energy performance, where future work will consider more economic factors.

Highlights.

• Overview of net-zero energy building (NZEB) development, research, strategies, and implementation targets

• Renewable energy sources: solar photovoltaic (PV), solar PV/T, solar thermal, wind, biomass, combined heat & power

• Infrastructure connections: electric grid, district heating/cooling, energy storage

• Energy-efficiency measures: building envelope design, HVAC, hot water, phase-change material

Abbreviations

ASHP

air-source heat pump

ASHRAE

American Society of Heating, Refrigerating, and Air-Conditioning Engineers

BAPV

building-applied PV

BIPV

building-integrated PV

CCHP

combined cooling, heating, and power

CHP

combined heat and power

COP

coefficient of performance

DD

dedicated dehumidification

DHW

domestic hot water

DOE

U.S. Department of Energy

EV

electric vehicle

ERV

energy-recovery ventilator

GHG

greenhouse gas

GHX

ground heat exchanger

GSHP

ground-source heat pump

GWP

global warming potential

HPWH

heat pump water heater

HRV

heat-recovery ventilator

HVAC

heating, ventilation, and air-conditioning

LBNL

Lawrence Berkeley National Laboratory

LCC

life cycle cost

NIST

U.S. National Institute of Standards and Technology

NZEB

net-zero energy building

ORC

Organic Rankine Cycle

PEMFC

proton exchange membrane fuel cell

PCM

phase-change material

P/H

electrical-power-to-thermal output ratio

PPD

percentage of people dissatisfied (uncomfortable)

PV

photovoltaic

PV/T

photovoltaic/thermal

SOFC

solid oxide fuel cell

SNG

synthetic natural gas

U.S.

Unites States of America

V2H

vehicle-to-home

Appendix

Fig. A1 shows the research methodology of this work.

Fig. A1.

Flowchart of the research methodology