Table 1.

Comparisons between the technologies used for waste heat recovery.

S/No	WHR technology	Advantages	Disadvantages
1	Metallic recuperators	✓ Suitable for low-temperature applications.	✓ Not suitable for temperatures above 1,093 °C, ✓ Not suitable for applications associated with dirty exhausts.
2	Ceramic recuperators	✓ Used for high-temperature applications which operate within the range of 982 and 1,538 °C for the cold and hot sides respectively.	✓ Not suitable for applications associated with dirty exhausts.
3	Recuperative burners	✓ More compact and simpler concerning design and construction than standalone recuperators. ✓ Ease of retrofitting and its associated lower costs give it an advantage in terms of choice for WHR over standalone recuperators. ✓ Gap flow recuperative burner has lower exhaust gas emissions with the burner achieving almost similar efficiency as a regenerative system using a recuperative design.	✓ Exhibits a lower heat exchange area, hence their ability to recover energy is lower.
4	Regenerators	✓ Used for high-temperature applications which operate within the range of 982 and 1,538 °C for the cold and hot sides respectively. ✓ Suitable for applications associated with dirty exhausts.	✓ Has a large size, presupposing higher capital costs when compared to recuperators.
5	Regenerative burners	✓ More compact and simpler in terms of design and construction than a self-sustaining regenerative furnace. ✓ Ease of retrofitting and lower costs make its choice for WHR an attractive one. ✓ A slab-type heating furnace integrated with regenerative burners using a larger heat exchanger surface area with a large number of small recuperators has higher thermal efficiency and better fuel savings compared to a furnace with a conventional burner without a regenerator. ✓ Achieves a better reduction in the emission of CO2 and NOx.	✓ Has a lower mass than a self-sustaining regenerator, which results in a lower WHR ability.
6	Heat wheels	✓ Very suitable for applications ranging from low to medium-temperature. ✓ Suitable for gas-to-gas applications	✓ Not suitable for high-temperature applications which compromise the strength and reliability of the duct wheel’s air seals. ✓ Has difficulty in preventing cross-contamination that may occur between two gas streams.
7	Passive air preheater-plate-type	✓ Suitable for applications ranging from low to medium temperatures. ✓ Less susceptible to contamination than HWs.	✓ Bulkier, more expensive, and more susceptible to the problem of fouling than HWs.
8	Passive air preheater-heat pipe-type	✓ Suitable for applications ranging from low to medium temperatures ✓ Does not experience cross-leakage between the supply air and exhaust gas. ✓ Compactness of heat recovery, absence of moving parts, attractive costs and reliability.	✓ Not suitable for use in cases where the exhaust is associated with high sulphur content, due to condensation at the surface and corrosion.
9	Economizers	✓ Suitable for low to medium-temperature applications. ✓ Improves boiler efficiency. ✓ Reduces fuel consumption ✓ Suitable for application in industrial systems with acidic and dirty exhausts. ✓ Can recover WH in the range of 10–25%.	✓ Condensation of flue gas moisture takes place on the economizer tubes when the feed water temperature is not sufficiently high which may absorb CO2 and SO2 to form an acidic solution that corrodes the tubes. ✓ Experiences the problem of oxygen pitting, caustic embrittlement, due point corrosion and hydrogen attack.
10	Waste heat boiler	✓ Suitable for medium to high-temperature applications ✓ Waste heat boilers of fire-tube types have the advantage of being simple to construct, install and maintain. ✓ A waste heat boiler of water-tube type can cope with the elevated pressures of steam than a fire-tube boiler. ✓ Water-tube types can respond quickly to heat input changes.	✓ Fire-tube types cannot operate at higher pressures ✓ Water-tube types are more difficult to design install and maintain.
11	Heat pumps	✓ Ability to upgrade WH quality to a value that may be more than two times the energy it consumes. ✓ Suitable for applications where heating and cooling are required simultaneously. ✓ The absorption heat pump type has more applications than the compression type because of its flexible principle of operation and adjustable cycle structure. ✓ The absorption heat transformer (AHT) type has the capability of upgrading the temperature of WH streams while using only a negligible amount of electrical energy and no additional primary energy ✓ A triple-state AHT (TSHT) is suitable for industrial applications where the temperatures of heat energy are over 200 $°C$. It is used to achieve high gross temperature lift.	✓ The single-stage AHT (SSHT) model is only capable of achieving small temperature increments up to about 50 $°C$ ✓ The double stage AHT (DSHT) version, which is an advanced type of the single stage can achieve more improved temperature increases just up to 80 $°C$ ✓ Many applications in the industry require temperatures of heat energy of more than 200 $°C$ which cannot be achieved using either SSHT or SSHT.
12	Organic Rankine cycle	✓ Organic Rankine cycle (ORC) shows high potential for low-grade waste heat (LGWH) applications. ✓ Very good in reducing fuel consumption. ✓.Due to its simple design and ability to recover LGWH, the ORC is considered better than the Kalina and steam Rankine cycles as well as many other technologies used in WHR. ✓ Due to the requirement of lower optimum pressure for its operation compared to Kalina cycles, ORCs demand lower materials and sealing costs.	✓ Associated with flammability risk while using some working fluids (hydrocarbons) for high-efficiency WHR.
13	Kalina cycle	✓ Can effectively extract low-temperature heat due to the non-isothermal phase change behaviour of the zeotropic mixture of ammonia and water. ✓ Achieves efficiency that competes with that of the ORC for WH temperature of 200 $°C$ and above.	✓ Its heat recovery steam generator requires a high steam fraction which causes a lower overall coefficient of heat transfer and a larger heat exchange area. ✓ The system is prone to corrosion. The air or CO2 that are impurities in liquid NH3 can cause mild steel-stress corrosion cracking.
14	Goswami cycle	✓ Has the advantages of the production of simultaneous power and cooling effect within a cycle; design flexibility; high efficiency in the conversion of LGWH, and improved resource utilization possibility when compared to separate systems.	✓ Still in the research stage.
15	Trilateral flash cycle	✓ Trilateral flash cycles (TFCs) are suitable for the conversion of LGWH in the range of 70–200 °C. ✓ Has shown huge prospects of not only competing favourably with ORCs but also offering the advantage of a higher potential for WHR and better outputs per unit of heat input.	✓ The absence of suitable two-phase expanders with high isentropic efficiencies is the reason TFCs are yet to experience appreciable success.
16	Supercritical CO2 Rankine cycle	✓ Has shown very high potential for LGWH recovery ✓ The advantage CO2 has over some other substances is that it is non-toxic, non-flammable, cheap, and non-explosive; and has attractive critical pressure and temperature of 73.8 bar and 31.1 °C, respectively. ✓ The cycle’s heating process does not go through a distinct two-phase region in a similar way that is observed in conventional steam Rankine cycles, thereby achieving a thermal match that is better in the boiler with a lower case of irreversibility. ✓ Has the capability of generating more power and higher efficiency than the ORCs.	✓ The barrier associated with its application is that CO2 has a low critical temperature (31.1 °C) which is a limitation on the process of condensation. This is because CO2 is required to undergo cooling below 31.1 °C (say 20.8 °C) to condense. However, cooling the CO2 below ambient temperature conditions introduces design complexities of the cooling system, thus necessitating the consideration of alternative working fluids such as organic ones.
17	Combined heat and power systems	✓ Among all the power plants running on fossil fuel, the Combined Cycle Gas Turbine (CCGT) system working in the mode of combined heat and power (CHP) is considered the most effective means of exploiting the fuel's energetic potential, thus making it the main development directions of future thermal power generation. ✓ Has high thermal efficiency and minimal emissions of CO2. ✓ In comparison to separate electricity or heat-generating systems, cogeneration is capable of providing energy savings in the range of 15–40%. ✓ Offers a cost-effective means of reducing GHG emissions and has higher flexibility compared to other power generation systems such as single Joule or Rankine Cycles.	✓ High investment costs ✓ Prone to heat losses to the ground ✓ The heat generated during summer periods may go to waste ✓ Combustion in the cities is the main cause of premature deaths due to excessive air pollution, etc.
18	Absorption refrigeration system	✓ Easy to control, operationally reliable and simple structurally. ✓ Has higher thermal efficiency and output power compared to other configurations. ✓ Reduction in fuel consumption and CO2 emission in vehicles.	✓ Bulkier in size, associated with lower values of COP and takes longer time to produce the same refrigerating effect as compared to vapour compression cycles.
19	Thermoelectric Generators	✓ A convenient and promising technology that converts WH into electrical energy, without degrading the environment or generating noise. ✓ Associated with reduced costs of maintenance and operation due to the absence of moving parts. ✓ Fuel consumption reduction.	✓ Incapable of realizing $ZT$ of more than 1, providing the highest realistic efficiency of only about 5% for the conversion of WH of exhaust gas in automotive applications.
20	Turbocompounds	✓ For high engine loads, the device can significantly reduce fuel consumption during operation. ✓ Reduction in CO2 emission in vehicles ✓ Exhibits high potential for power improvement when linked with an EGR valve.	✓ For low engine loads, turbo compounds may even have a negative impact as they can cause a slight increase in fuel consumption.