4.1. Recuperators

Recuperators are used to recover WH from flue gases within medium to high-temperature ranges, and they find application in melting furnaces, radiant tube burners, afterburners, etc. [18]. They are used as ancillary devices in gas turbines to improve energy efficiency. Gas turbine recuperators employ the WH of flue gases at high temperatures to preheat the mixture of air and fuel going into the combustion chamber. The incorporation of a heat recovery steam generator for further heat extraction from the flue gas in a conventional steam RC has been found to produce about 50% more electrical energy without more input fuel in the combined cycle plant [1]. Recuperators are generally categorized into radiation, convection, or a combination of the two. A different type of recuperator that combines the advantages of both the convective and radiation type recuperators possesses a bigger shelf with smaller tubes while the cold air flows around the shelf as shown in Fig. 2.

 

Fig. 2

A combination of the radiative and convective recuperator [2].

The material used in constructing recuperators may be either ceramics or metal. In applications of WHR with temperatures below 1,093 °C, recuperators made of metals are considered, whereas, for temperatures above this, recuperators made of ceramics are used. High-temperature applications operate within the range of 982 and 1,538 °C for the cold and hot sides, respectively [26].

4.2. Regenerators

Regenerators are heat exchangers that intermittently store the heat energy from a high-temperature fluid within a thermal storage medium before its transfer to a colder fluid. The design of a furnace regenerator is such that cold and hot air flow alternately via two chambers made of bricks. For high-temperature applications associated with dirty exhausts, regenerative systems are employed. However, the major demerit of regenerators is mainly due to their large size, presupposing higher capital costs when compared to recuperators [26]. A schematic diagram of a furnace regenerator is shown in Fig. 3.

 

Fig. 3

Schematic diagram of a regenerative furnace [18].

The system comprises two burners containing refractory materials like aluminium oxide. The flue gas is used in heating this material and when completely heated, there is a reversal in the direction of the flue. The airflow is adjusted such that the incoming air for combustion first passes through the hot burner, and thereafter enters the furnace. The two burners alternate periodically, where one extracts heat from the flue gases, while the other transfers heat to the combustion air. Similarly, rotary regenerators referred to as heat wheels (HWs) facilitate the transfer of heat by storing energy and alternating the flow of cold and hot fluids through the system. It uses a porous disc made of a material of high thermal capacity spinning between the two ducts and causing heat exchange between the hot and cold gas ducts. This technology is cost-effective and suitable for gas-to-gas applications. A schematic diagram of a rotary regenerator is shown in Fig. 4.

 

Fig. 4

Schematic diagram of a rotary regenerator (heat wheel) [18].

HWs particularly are very suitable for applications ranging from low to medium-temperature due to the occurrence of thermal stresses in high-temperature applications. High-temperature differences between the cold duct and the hot duct can cause differential expansion that leads to large deformations, thereby compromising the strength and reliability of the duct wheel’s air seals. However, it is interesting to note that in cases where applications of high temperatures are unavoidable, ceramic wheels can be employed as an alternative measure. Another challenge associated with the application of HWs is the difficulty in preventing cross-contamination that may occur between the two gas streams, as there is the possibility of movement of contaminants in the porous material of the wheel [18]. HWs are applied to space heating and processes of air conditioning.

4.3. Passive air preheater

A passive Air Preheater (PAP) is a WHR device in which there is an exchange of heat between two gaseous fluids for applications ranging from low to medium temperatures. The problem of cross-contamination between the two gas streams is not experienced here, therefore, the device finds application in the exhaust of gas turbines, ovens, steam boilers, etc. PAPs are classified into– the Heat Pipe (HP) and the plate-type. The plate type comprises multiple parallel plates, which produce separate channels for both streams of cold and hot gases. The cold and hot fluids flow alternately between the plates, providing a significant heat transfer area. This device has the advantage of being less susceptible to contamination than HWs. But the device is bulkier, more expensive, and more susceptible to the problem of fouling [18]. The other type-heat pipe (HP), as a heat exchanger is an important heat recovery device, which does not experience cross-leakage between the supply air and exhaust gas. HPs have numerous advantages, like the compactness of heat recovery, absence of moving parts, relative economy, small pressure drop associated with the air side, reliability, as well as total separation of cold and hot fluids. The HP among other applications is mostly employed in the recovery of heat from exhaust gases in a furnace stack [27]. Fig. 5 shows the working cycle of the HP preheater.

 

Fig. 5

PAP - HP working cycle [28].

Jouhara et al. [2] conducted a test on WHR using a Heat Pipe Heat Exchanger (HPHE) in a ceramic kiln (with varying operational conditions) for a period over 27 h; thus, achieving heat recovery in the range of 76–103 kW. The value of the overall heat recovered per annum was 876 MWh, whereas the return on investment for the system was 16 months with an annual savings of 30,000 GBP. The gas-to-water HPHE was installed by Ref. [29] in Ireland to proffer solutions to a problem associated with a dairy factory, although the sulfur content of the exhaust fired from heavy fuel oil was relatively high in sulfur, therefore, not suitable for use due to condensation at the surface and corrosion. The installation of three HP gas-to-water units was carried out, with the achievement of WHR of 488 kW, thus generating savings of about EUR 87,000/yr. with a PBP of 19 months. In the year 2020, the Horizon project developed a standardized flat plate HP for recovering heat from radiant WH sources with smooth application and minimal cases of disruption of the process [20]. The device has the advantage of the capability for two-phase heat transfer. Another advantage is the flexibility for space requirement and heat transfer area minimization, as well as attractive costs.

4.4. Regenerative/recuperative burners

It is interesting to note that some designs of burners integrate either recuperative or regenerative principles. In comparison to standalone recuperators or regenerative furnaces, these burners are more compact and simpler in terms of design and construction. Their energy efficiency is higher when compared to burners working with ambient air. A standalone recuperative burner as an aspect of its design makes use of heat exchange surfaces. The reason is to enable the body of the burner to recover as much energy as possible from the exhaust gas passing through the body. A standalone regenerative burner allows the passage of exhaust gases through the body of the burner into a refractory media case, operating in pairs in a manner that is similar to a regenerative furnace. Usually, a recuperative burner exhibits a lower heat exchange area, while a regenerative burner exhibits a lower mass than self-sustaining units. Therefore, their ability to recover energy is lower. However, their ease of retrofitting and their associated lower costs make their choice for WHR an attractive one. The schematic diagrams of the principles of regenerative and recuperative burners are shown in Fig. 6a and b, respectively.

 

Fig. 6

(a) Principle of a regenerative burner [30] (b) Principle of a recuperative burner [2].

Jang et al. [31] carried out a study on a slab-type heating furnace integrated with regenerative burners and recorded a thermal efficiency of 72%. The system achieved fuel savings of 15% compared to the furnace with a conventional burner without a regenerator. Working with a double regenerative radiant tube burner, Zhong et al. [32] concluded that its efficiency was higher than the efficiency of a conventional radiant tube burner. Thus, the regenerative burner achieved a better reduction in the emission of CO₂ and NO_x. Similarly, Wunning [33] recorded lower exhaust gas emissions using a newly developed gap flow recuperative burner. The burner achieved almost similar efficiency as a regenerative system using a recuperative design. This was done using a larger heat exchanger surface area with a large number of small recuperators.

4.5. Economizers

A finned tube heat exchanger also referred to as an economizer is a device designed for the exchange of heat, and used for applications within low to medium temperatures. It consists of a circular tube to which fins are attached to increase its heat transfer surface area. A finned tube-type heat exchanger captures the WH of flue gases, using it to heat liquid fluids. This device finds application in the production of home hot water, hot process liquid, hot water for process heat, etc. The mode of operation is such that the liquid passing through the tubes is heated by the exhaust gases flowing across the tubes [18].

The Horizon 2020 project also developed the Heat Pipe Condensing Economizer (HPCE), which integrated the advantages of both heat pipes and economizers [20]. According to Christodoulides et al. [20], the potential market for the HPCE is the entire market for industrial and commercial boilers (ignoring their built-in condensing economizers). The suitable areas of application for HPCEs are industrial systems with acidic and dirty exhausts such as petrochemical industries, etc. [20]. Other potential areas of application are the food, cement, glass, and steel industries. It is also pertinent to note that condensing economizers can recover WH in the range of 10–25% more than the noncondensing types, thus making their primary applications fall within expensive primary fuels [20]. It can also find applications in concentrated-type solar collectors and nuclear power plants.

Economizers employed for the boiler system were able to improve the efficiency by about 1% for every 5 °C exhaust gas temperature reduction; thus, showing the possibility of achieving a 5–10% reduction in the system’s fuel consumption within less than two years of PBP [2,34]. The report of another study conducted by Maxxtec [35] revealed that irrespective of the system’s design, a reduction of the flue gas temperature by 140 °C is capable of reducing fuel consumption by 7%. There are several designs of economizers available (coiled tubes, condensing and non-condensing, and finned tube economizers) which are used for different applications [2]. The condensing and non-condensing designs find application in the improvement of the boiler system’s efficiency, while the other designs are employed in flue gas WHR involving large processing facilities and thermal power plants [2]. The finned tube heat exchanger is shown in Fig. 7. Condensation of flue gas moisture takes place on the economizer tubes when the feed water temperature is not sufficiently high which may lead to the absorption of CO₂ and SO₂ gases, and the eventual formation of an acidic solution that corrodes the tubes. Other challenges associated with economizer tubes are oxygen pitting, caustic embrittlement, due point corrosion and hydrogen attack.

 

Fig. 7

Schematic diagram of finned tube exchanger/boiler economizer [18].

4.6. Waste heat boiler

A waste heat boiler (WHB) is a two-pass boiler that is employed in applications ranging from medium to high temperatures. This water tube boiler is used in the production of steam through the exchange of energy between the exhaust gases and the cold water contained in the parallel tubes as depicted in Fig. 8.

 

Fig. 8

Schematic diagram of a WHB [18].

Besides, in cases where the WH is enough to generate the required steam output, auxiliary burners or afterburners are made available to actualize the desired objective [36]. The steam produced may be utilized for process heating or power generation. In most applications of recovered WH, the transfer of heat is continuously made to a cold fluid from a hot one. Applications of WHR in industrial processes have a general rule of thumb, which considers hot fluids at temperatures lower than 120 °C as the limit for WHR because of the risk of condensation of liquids that are corrosive [36]. Besides, the incessant rise in the prices of fuels has made it inevitable to consider the recovery of WH from fluids at temperatures below 120 °C for space heating and some low-temperature WH applications.

4.7. Heat pumps

A heat pump operates by changing the heat flow direction, and mainly works on the Vapour Compression Cycle (VCC) principle. In a VCC, the circulating fluid is separated physically from the WH source and the user (heat to be used in the process) streams and is re-used cyclically. The user properly so called is the heat to be used in the process. A compression heat pump (CHP) as shown in Fig. 9 is employed for space heating if low-temperature energy from water, ambient air, etc. is raised to the temperature of the heating system by doing some work on the fluid with the aid of a compressor.

 

Fig. 9

Schematic diagram of a compression heat pump [36].

A heat pump can upgrade WH quality to a value that may be more than two times the energy which is consumed by the device. Chen et al. [37] stated that the recovery of WH using heat pump technology offers a significant reduction in heat pollution to the environment. There is an increase in heat pump applications, especially in industrial processes due to its capability of recovering LGWH and upgrading it for the production of process steam [36]. Heat pumps are also applied in situations where heating and cooling are required simultaneously. Typical cases of this can be found in plastics factories where the cold water leaving a heat pump is used for the cooling of the injection-moulding machine, while the output heat of the heat pump is utilized for office heating. The device is also used for the drying of products, the drying of compressed air, as well as the maintenance of a dry atmosphere for storage purposes.

A significant reduction in demand for industrial WH and high-quality steam can be actualized through the integration of heat pumps. Liew and Walmsley [38] recorded energy cost reductions of 343,859 and 168,829 USD/year using two-stage thermal vapour compression and four-stage mechanical vapour compression systems, respectively. An application of a hybrid (compression/absorption) heat pump using a mixture of water and ammonia as its WF was carried out by Nordtvedt et al. [39] in the food industry. The hybrid pump recovered WH from the refrigeration system at about 50 °C and delivered hot water at 90 °C.

An absorption heat pump (AHP) is another type of heat pump whose technology is different in comparison with a CHP. In an AHP, the electrical compressor is replaced with two components capable of raising the pressure with just 5% of the compressor’s energy under the same operating conditions [40]. The AHP has more applications than the CHP because of its flexible principle of operation and adjustable cycle structure. The AHPs may be classified based on different purposes as type I for increasing the capacity of heating power, and type II -absorption heat transformer (AHT) for raising the temperature of a heat flow [41,42]. According to Ref. [42], LiBr-water and water-ammonia are the main employed working pairs. AHTs are highly suitable for industrial WHR as it has the capability of upgrading the temperature of WH streams while using only a negligible amount of electrical energy and no additional primary energy.

AHTs are of different types, namely single-stage heat transformers (SSHT), double-stage heat transformers (DSHT) and multi-stage heat transformers, such as triple-state heat transformers (TSHT). The SSHT is capable of achieving small temperature increments up to about 50 °C, which is only suitable for cases of small temperature increases as seen in water purification plants [43]. However, the DSHT, which is an advanced type of SSHT is used to achieve more improved temperature increases up to 80 °C, while maintaining the coefficient of performance (COP) of about 0.36 [44]. Essentially, the DSHT is a combination of two SSHTs. To upgrade the gross temperature lift of a DSHT, the concept of a multi-compartment and generation was proposed by Ji and Ishida [45], wherein the absorber of the cold cycle was connected to the hot cycle's evaporator. The use of two and four-compartment absorbers in the hot and cold cycles caused the exergetic coefficient of performance to increase by 6.81%, and the gross temperature lift by 10.7 °C. However, there was a reduction in the cycle’s COP from 31.44% to 29.34% as a result of the improved gross temperature lift.

SSHTs and DSHTs are only able to achieve gross temperature lifts of roughly 50 °C and 80 °C, respectively. However, many applications in the industry require temperatures of heat energy that are more than 200 °C, which cannot be achieved by either SSTHs or DSHTs. A situation such as this requires a more advanced system like the TSHT [46]. A TSHT comprises nine basic units, viz, a generator, an evaporator, a condenser, an absorber, two absorber-evaporators (operating at different temperatures), and three heat exchangers. However, only a scanty number of investigations have been carried out on the TSHTs. Zhuo and Machielsen [47] showed that a TAHT is capable of achieving gross temperature lift values of 145 °C while maintaining a COP of roughly 0.2. The study conducted by Lee and Sherif [48] demonstrated that TAHTs achieved lower COPs and ECOPs but higher gross temperature lifts than both SSHTs and DSHTs.

The data obtained by Cudok et al. [41] from three major manufacturers of AHTs have revealed a reliable perspective on the level of technology of commercial products and operative installations. From the information obtained, there were only 48 installations of AHTs in 42 plants from 1981 to 2019 with a total capacity of approximately 134 MW. Over 74% of the AHTs were installed in Asia, and about 61% of them were applied in the chemical industry. The economic importance in terms of the feasibility of the AHT installation is based mainly on the ability to consume electricity at only about 1% of its capacity.

Many positive case studies have been reported in the literature concerning AHTs in terms of their capability of recycling appreciable amounts of waste energy during operation, but their commercialization has hugely suffered a low level of implementation. The low implementation of AHTs is partly a result of a lack of interest in the energy savings that can be achieved using WHR technologies, in addition to insufficient knowledge of the effects of CO2 emissions, coupled with the problem of inadequacy regarding the number of specialized technical experts to maintain and certify the reliability of AHTs. Other factors such as high equipment capital costs associated with AHTs may be the reason why many investors are not attracted in the least [49]. The general rule of thumb is that if a piece of equipment does not offer significant returns upon investment with attractive payback periods, they are likely not to be considered as an investment target.

Despite the high cost of investment of AHTs (about 1500 $/kW), it has been established that their use for WHR is to a large extent superior to that of a conventional heat pump whose investment cost is approximately 900 $/kW [50].

In all, the economic evaluations available in the literature show that there is hope for the AHT technology market as the return on investment gathered from previous studies on many of the already installed facilities presents an encouraging economic outlook [41].

4.8. Rankine cycle/organic rankine cycle

The Rankine Cycle (RC) is a typical example of an ideal steam-power cycle. It causes the vaporization of a WF subjected to high pressure, and thereafter allows the expansion of the WF in a turbine. The power produced through the expansion process in the turbine drives a generator for electricity production [51]. An RC that uses steam as a WF is applied in many recent power plants. RCs operating on WH capture the WH generated from a given number of industrial processes, especially facilities that use fuel combustion. The generated power can be utilized to produce electricity to run the vehicle’s electrical gadgets or may be used in augmenting the vehicle’s propulsion system [52].

Many of the conventional techniques of WHR are not efficient for converting the LGWH of flue gases to electricity. However, the emergence of the Organic Rankine Cycle (ORC) proved to be a promising one, as the device exhibited great potential for LGWH applications. Among the numerous thermodynamic cycles employed for LGWH conversion to power, the ORC is the most commercially developed for small and large-scale power plants [53]. The ORC uses the steam RC principle, but employs organic WFs with low boiling points instead of steam, for heat recovery from a heat source of lower temperature. The ORC consists of an expansion turbine, a condenser, a pump, and a boiler as shown in Fig. 10.

 

Fig. 10

Schematic diagram of an ORC [54].

Furthermore, whenever there is a need for superheat, a superheater is usually provided. It is interesting to note that the ORC has been under extensive study for many years now. And the pronounced attention given to the Rankine bottoming cycle caused some key players in the automotive industry to initiate an investigation of its potential [55]. A given number of researchers have shown how the ORC was used for the achievement of more than 10% fuel consumption reduction in passenger vehicles as well as trucks used for commercial purposes. Vaja and Gambarotta [56] conducted a performance investigation of a stationary Internal Combustion Engine (ICE) with ORC and achieved a 12% increase in efficiency. Mojtaba [57] carried out research on two ORCs of different configurations with the ability to simultaneously recover WH from exhaust gas and coolant of a diesel engine of 12 L capacity. This was aimed at optimizing the process to maximize the production of power and the cycle’s thermal efficiency. Due to its simple design and ability to recover LGWH, the ORC is considered better than the Kalina and steam RCs as well as many other technologies used in WHR [58]. However, the flammability risk associated with the ORC fluids (hydrocarbons) used for high-efficiency heat recovery should necessitate the investigation of new WFs to ensure compliance with established pollution control measures and regulations [58]. Hydrocarbons as WFs for ORCs have explosive characteristics not withstanding their low costs, good environmental compatibility and high efficiency in waste heat recovery. Hence, it is worthwhile to search for working fluids, which are as efficient as hydrocarbon fluids but are not associated with risks of flammability and other disadvantages such as high cost, environmental incompatibility, etc.

The ORC technology was employed by Casci et al. [59] for medium-temperature exhaust gas WHR in a ceramic firing process using a tile-tunnel kiln, in a production plant for ceramics. A heat transfer fluid (thermal oil) was used for transferring heat to the ORC engine. The overall efficiency obtained approached 80%. Using some real data obtained from the cement industry, Ahmed et al. [60] produced and investigated a design for an ORC in a gas turbine for converting the WH of the gas turbine into electricity. It has been reported that the ORC’s effectiveness using R134a as the WF can go up to 93% [20]. For an option of 7000 h/yr operational period, the economic analysis carried out showed maximum and minimum PBPs of 4 and 2.9 yrs, respectively, whereas an operational period of 8000 h/yr, gave the maximum and minimum PBPs of 3.5 and 2.5 yrs, respectively [20]. A comparative thermodynamic study of the Kalina cycle and ORC was carried out by Nemati et al. [61] for a cogeneration system, and the results obtained showed that the ORC required an optimum pressure value that was considerably lower than that of Kalina, thus showing lower materials and sealing costs of the ORC.

4.9. Kalina cycle

In principle, the Kalina Cycle (KC) which is known as reversed absorption cycle, is a modification of the RC [62]. The major difference between the KC and the RC is the fact that it uses a mixture of substances as its WF, instead of a pure fluid such as water. The WF mixture is composed of two or more substances, especially ammonia (NH₃) and water (H₂O) [53]. KCs can effectively extract low-temperature heat due to the non-isothermal phase change behaviour of the zeotropic mixture of ammonia and water. The WH is utilized in vaporizing a significant portion of the WF with the aid of the Heat Recovery Steam Generator (HRSG) as shown in Fig. 11. Owing to the low temperature of the WH, a separator is placed after the HRSG to separate the steam from the liquid before admission into the turbine. It is noteworthy that the KC has its shortcomings. The HRSG requires a high steam fraction which causes a lower overall coefficient of heat transfer and a larger heat exchange area. Another disadvantage is that the system is prone to corrosion. The air or CO₂ which are impurities in liquid NH₃ can cause mild steel-stress corrosion cracking. In addition, NH₃ is also a substance that is highly corrosive to zinc and copper [63]. Numerous KC designs share some similarities with the RC, with each design having an application that is specific to different sources of heat as experienced in gas to combined cycles or low-temperature geothermal plants [64]. KCs using WH recovered from a steel plant can produce more than 3 MW of power [62]. Mahmoud et al. [65] estimated net output power of 7.35 MW and a PBP of 21 months using a KC with combined WHR in a cement plant. Zhu et al. [66] adopted a cascade utilization of the source of heat using two evaporators in series and recorded 26.4% power recovery efficiency.

 

Fig. 11

Schematic diagram of the KC [63].

The concept of the KC is increasingly being deployed due to its ability to utilize LGWH within the range of 100–200 °C and the zeotropic nature of the WF. However, the RC has been shown to have a better performance for certain temperature ranges of the WH. From the comparative study conducted by Milewski and Krasucki [67], the efficiency of the KC competed with that of the ORC for WH temperature of 200 °C and above.

To obtain an improved output power from the WH source, efforts have been made toward the integration of the Kalina and the ORCs. Oksel and Koc [68] conducted a study on a combined KC and ORC using exhaust gas WH from a CHP engine. The ORC with a heat exchanger was added between the turbine and the condenser of the KC as shown in Fig. 12.

 

Fig. 12

Combined KC and ORC using WH from the CHP engine [68].

An ammonia-water (NH₃–H₂O) mixture was used as the WF in the KC while R123 was selected for the ORC. The exhaust gas of the CHP engine was cooled from 450 to 120 °C while converting its energy to electricity in the KC, thus hindering the performance of ORC. The system generated maximum power output of 168.69 and 42.34 kW for the KC and ORC respectively. The combined cycles recorded a thermal efficiency of 26.5% with a PBP of 4.2 years. Similarly, He et al. [69] applied a combined ORC and KC in the WHR of an ICE. The ORC recovered the WH of the high-temperature flue gas and that of the lubricant while the KC recovered the WH of low-temperature cooling water. The system recorded an overall efficiency of 20.83%.

4.10. Goswami cycle

This thermodynamic cycle which is referred to as the Goswami cycle was proposed by Goswami [70]. The device uses a binary mixture to produce power and refrigeration simultaneously in one loop. It integrates an RC with an absorption cooling cycle. According to Vijayaraghavan and Goswami [71], the 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. This device is still in the research stage. The experimental test rig was installed around the late 90s at the University of Florida, USA.

4.11. Trilateral flash cycle

Another important thermodynamic cycle that uses an ammonia-water mixture as a WF is the trilateral flash cycle (TFC). The expansion process of the cycle starts from the saturated liquid state rather than the vapour phase. The heat transfer from a source of WH to the WF (liquid) is done with near-perfect temperature matching by avoiding the boiling part. The NH₃–H₂O mixture gives a more suitable match with the temperature profiles in the heat sink. A schematic diagram of the TFC is shown in Fig. 13.

 

Fig. 13

Schematic diagram of a TFC [20].

In TFCs, the WHR potential depends largely on the efficiency of the two-phase process of expansion [53,72]. The major disadvantage of TFCs is the unavailability of suitable two-phase expanders with high isentropic efficiencies. Hence, the reason TFCs are yet to experience appreciable success. Extensive investigations of this device carried out in the 1970s, reported adiabatic efficiencies in the neighbourhood of 50% for twin screw expanders (Lysholm-type) [53]. Studies have revealed that it is possible to design and construct twin-screw expanders with more than 80% isentropic efficiencies. The performance evaluation conducted on some screw machines shows that two-phase expansion of fluid with an isentropic efficiency of more than 70% is possible [53].

TFCs with the capability of converting LGWH in the range of 70–200 °C into electricity have been developed [20]. The device has shown huge prospects of not only competing favourably with ORCs but also offering the advantage of a higher potential for heat recovery and better output per unit of heat input. TFCs are also capable of extracting heat from thermal energy sources where it is practically difficult for a Carnot cycle to produce power [73]. Results of several experimental investigations have revealed that TFCs exhibit high prospects as a technology to be further developed for LGWH recovery from thermal energy sources whose temperatures are below 100 °C [74]. Literature reports show that operational TFCs are yet to be a reality, for only a few units are available for demonstration.

4.12. Supercritical CO₂ rankine cycle

The supercritical CO₂ Rankine Cycle (SCRC) is a thermodynamic power cycle that has shown very high potential for LGWH recovery [75]. The cycle uses carbon dioxide and CO₂ (R-744) as a WF. The advantage CO₂ enjoys 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 [75,76]. WFs with low critical temperature and pressure have shown the capability of being directly compressed to their supercritical pressures; and through heating, can also be raised to their supercritical state before expansion. Chen [53] reported that the cycle’s heating process does not go through a distinct two-phase region in a similar way that is observed in conventional steam RCs, thereby achieving a thermal match that is better in the boiler with a lower case of irreversibility. An SCRC was suggested by Yamaguchi et al. [76] for electricity production with the use of solar energy. SCRCs use evacuated-type solar collectors to convert CO₂ into a high-temperature supercritical state. In Kyoto, Japan, a test rig of the device was developed and subjected to test under typical summer conditions. The test results revealed that the solar energy-powered RC using CO₂ as WF operates stably in the trans-critical region, and the time-averaged power production efficiency of 25% was recorded. However, the annual average efficiency for power production in a previous theoretical study was found to be 22% [76]. The dynamic modelling of an SCRC carried out by Olumayegun and Wang [77] showed that the system performs optimally when the turbine’s inlet temperature is varied according to the fluctuation in the source of waste heat.

Oyedepo and Fakeye [1] reported that an SCRC is capable of generating more power and higher efficiency than the ORC. However, the major barrier in its application is the fact that in a multi-phase flow at the trans-critical stage, the WF’s properties are neither clearly gas nor liquid, for which reason, thus lacks good expanders (with multi-phase compatibility) that are not affected by droplets of liquid and attendant erosion problems. CO₂ is selected as the WF for SCRC on the grounds of its low critical pressure, thus allowing lower pressures during operation. Other benefits of CO₂ are the fact that it is cheap, readily available, non-toxic, stable, and chemically unreactive within the system’s range of operating temperatures [1,78]. However, the barrier associated with the application of SCRC 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 [1]. However, cooling the CO₂ below ambient temperature conditions introduces design complexities concerning the cooling system, thus necessitating the consideration of alternative WFs such as organic ones [78].

4.13. Absorption refrigeration systems

WHR from IC engines has been predominantly applied to turbocharging and cabin heating using absorption chillers. Results of the performance evaluation of the device have shown its ability to enhance significantly the performance of the system depending on the part-load of the engine. Hugues et al. [79], stated that the technique could be employed for the refrigeration and air conditioning of vehicles. An experimental study on the adsorption Air-Conditioning System (ACS) installed in an ICE for the cooling of locomotive driver cabins has been carried out. The system uses the working pairs (zeolite-water) for its operation while sourcing energy from the exhaust gas WH of an ICE. Thus, providing continuous refrigeration to the driver’s cabin instead of doing so using an ACS powered by steam compression cycles. The results of the experiments carried out for the cooling of the locomotive driver’s cabin using a single absorber ACS showed that apart from being easy to control, the device is operationally reliable and is simple structurally [80]. An absorption refrigeration unit coupled to a Caterpillar engine is effective in the cooling of the charge air before its ingestion into the cylinder engine as well as in the processes of air conditioning. Reports in the literature show that an absorption combined cycle in a diesel engine with pre-inter cooling has higher thermal efficiency and output power compared to other configurations. However, the report by Talbi et al. [81] showed that the overall efficiency of an inter-cooler is higher than that of a pre-inter-cooling cycle.

4.14. Thermoelectric generator

Power plants traditionally convert heat energy into mechanical energy for the production of electricity. However, Thermoelectric Generator (TEG) has been developed to directly generate electricity from heat. A sizeable number of thermoelectric devices have been investigated to ascertain their WHR capability in automotive vehicles. TEGs are the most convenient and promising technologies that convert WH into electrical energy, as they do so without degrading the environment or generating noise [82,83]. They are associated with reduced costs of maintenance and operation due to the absence of moving parts. But they have low conversion efficiency which has posed to be a challenge to their globalization [84]. TEGs for the generation of electricity use the n- and p- type dissimilar semiconductor elements due to their ability to increase the output potential of the device [85]. Fig. 14 shows a typical TEG arrangement for electricity generation.

 

Fig. 14

Schematic diagram of a TEG [85].

A thermoelectric device that generated about 58 W at peak conditions using FeSi2 elements was developed in the late 80s. However, subsequent studies were conducted on TEGs for passenger car applications. And in the 90s, the results of research carried out showed improvements in the level of power produced as a record of up to 200 W was achieved in passenger cars or about 1 kW for a small truck application [86]. In WHR systems for combustion engines, the difference in temperature applied across the junction of the device comes from the difference between the hot exhaust gas temperature and the cooling fluid temperature. The factor, ZT (figure of merit), which is a dimensionless term is a characteristic constant for the two conductors used in the TEG. ZT and the WH source and sink temperatures (Thot and Tcold), are among the major determinants of the performance of thermoelectric materials [87]. Thermoelectric devices rely on the ZT for electrical performance. Eq. (2) demonstrates this relationship:

where S is the Seebeck coefficient, σ is electrical conductivity, and λ is thermal conductivity. σ is the reciprocal of electrical resistivity (ρ). The conversion efficiency, ηTEM of TEGs is calculated using Eq. (3):

where ηCarnot = Carnot efficiency (i.e., the maximum achievable heat recovery efficiency)

As ηTEM is related to ZT, good thermoelectric materials are expected to present a high value of Seebeck coefficient, low electrical resistivity, as well as low thermal conductivity [88]. Metals exhibit high values of thermal conductivity, low values of electrical resistivity, and low Seebeck coefficients, thus engendering low values of ZT; whereas the highest possible values of ZT are obtained with semiconductor materials. The most employed materials for thermoelectric applications are bismuth telluride, lead telluride, and skutterudite, which provide ZT s close to 1 [88,89]. According to Hussain et al. [90], modern commercial TEGs are incapable of realizing ZT of more than 1, providing the highest realistic efficiency of around 5% for the conversion of WH of exhaust gas in automotive applications. Gunnar [52] shows that an investigation carried out by the Ford Motor Company in 2009 studied the WHR potential of TEGs in gas-electric hybrid vehicles. The results showed that about 2.4% of the waste energy in the exhaust gas could be recovered for a city cycle. And the value increased to 5.7% for the highway cycle, as a result of the higher loads and the temperatures of exhaust gases that occur under these driving conditions. Baker and Shi [91] reported that a study conducted in 2009 by Honda revealed that the use of the system in a 2.0 L gasoline engine would reduce overall fuel consumption by less than 1%, while Stobart et al. [85] predicted yearly fuel-saving potentials of 3.9%–4.7% for a passenger car and up to 7.4% for a transit bus.

Recent research is focused on industrializing new thermoelectric materials with higher values of ZT. One of the main challenges restricting the achievement of materials that exhibit high ZT is the interdependence of S and λ, where an increase in the value of S causes a decrease in the carrier concentration, which in turn decreases the value of λ. Furthermore, the difficulties encountered in lowering heat losses during the evaluation of σ are considered one of the major challenges [88,92]. It is interesting to note that some researchers in recent times carried out the synthetization of some semi-conductor materials (copper selenide and phase-separated PbTe0.7Se0.3), and generated values of ZT > 2 [88,93].

The properties of thermoelectric materials depend on temperature, giving the highest figure of merit under a certain range of temperature. Therefore, to achieve optimum performance, recent researches consider the design and fabrication of thermocouples with materials that are segmented based due to the operating temperature, to withstand the required temperature of the heat source while attaining the highest figure of merit [88,94].

4.15. Turbo compounds

Turbo-compounding is used to extract more work from the exhaust gases, and the extracted work is added to the system output. An additional turbine is positioned downstream of the turbocharger and the exhaust gas expands in the turbine, thus reducing its enthalpy. When multiplied by the turbine’s efficiency, this enthalpy decrease gives the maximum work that can be obtained from the device. The difference between the device and turbochargers is that in turbochargers, the energy recovered is used to power a compressor, while the output energy of turbo-compounds is used directly to augment the propulsion system of a vehicle. However, in some cases, the output of the device is used to drive a generator that produces electricity for the vehicle. Hence, there are two types of turbo-compounds, namely mechanical and electrical turbo-compounds. Fig. 15 shows one possible configuration for a combustion engine with a mechanical turbo-compound. In turbo-compounds for the generation of mechanical energy, the reduction of the output speed of the turbine is achieved using gears for the speed to match the engine’s crankshaft speed. In addition, a fluid coupling is employed ultimately for the separation of the compound turbine from the crankshaft’s torsional vibrations to preserve the turbine and the high-speed gear set from being damaged [95].

 

Fig. 15

Configuration of a mechanical turbo-compound system in a combustion engine [52].

Conversely, the electrical turbo compound is not connected to the vehicle’s propulsion system. This is a huge advantage in that their speed can be controlled independently of the engine’s speed, thus avoiding the danger of having to operate the turbine inefficiently under off-design conditions. Turbocompounds find application mostly in aircraft and heavy-duty engines, where they have caused the reduction of fuel consumption by 3–5%, even though the reduction was achieved under varying operating conditions. For low engine loads, turbo compounds may even have a negative impact as they can cause a slight increase in fuel consumption. This happens due to the increase in the exhaust backpressure caused by the turbine, which results in increased pumping losses during the gas exchange in the combustion engine. However, it is pertinent to note that the increased exhaust back pressure helps to improve the rate of Exhaust Gas Recirculation (EGR) for a short-route EGR system. This is because the pressure gradient between the intake and the exhaust manifold increases.

The simulation carried out by Ismail et al. [95] on a diesel engine of 2 L with a series- turbo-compound, revealed that reduction of the brake-specific fuel consumption could be achieved by positioning the turbine behind the turbocharger. This arrangement, nevertheless, increased the backpressure of the engine. Besides, it was shown that a turbo-compounding unit exhibits high potential for power improvement when linked with an EGR valve as the system generated an additional power of about 14 kW in consideration of isentropic expansion [95,96]. Experimental studies were conducted by Ref. [97] on a parallel turbo compound using an IVECO engine (3 L) of two exhaust configurations of which one was with the original Variable Turbine Geometry Turbocharger (VTGT), and the other linked with a new Fixed Geometry Turbocharger (FGT). The VTGT configuration showed promising results at low engine loads; whereas, at high engine loads, the advantages were minimal when compared with the FGT. However, the two systems showed the potential to recover power in the range of 2–2.5% [94]. Wasselin et al. [98] conducted a comparative study on a turbo compound installed in both a turboshaft engine (used for light rotorcraft) and a diesel ICE. The results of the investigation showed that the diesel ICE offered a better potential to reduce both CO₂ emissions and fuel consumption for both high and medium loads as well as for long-range missions. The fuel consumption in the range of 25%–50% in comparison to the turboshaft engine was dependent on power requirements [96].

An investigation of the three modes of turbo compounds, namely parallel, series, and electric turbo compounds was carried out by He and Xie [99] using steady-state simulations at varying engine speeds and loads, and at three driving cycles. The results obtained revealed that the electric turbo compound achieved the highest level in both power gain and Brake Specific Fuel Consumption (BSFC) reduction (above 7%) at both high engine speeds and loads for the varying driving cycles; whereas the parallel turbo compound gave the lowest degree in both power gain and BSFC reduction. However, all the turbo compound types performed well at high engine speeds and loads. According to Douadi et al. [100], the addition of turbo generators to automobiles is capable of increasing their efficiency in the range of 3–10%.

4.16. Protracted-finned counter flow heat exchanger

Another innovative heat exchange technology designed for effective WHR and reduction of engine emissions is the Protracted-Finned Counter Flow Heat Exchanger (PFCHE). This device was designed, fabricated and investigated experimentally by Ravi and Pachamuthu [101]. The results obtained showed that the PFCHE has a higher capacity to extract waste heat compared to the same heat exchanger without fins. The PFCHE was able to recover 550 W of waste heat when the fin length varied from 0.6 m to 1.0 m, and the number of fins was increased from 6 to 12; whereas it only recovered 335 W of heat in the absence of fins (Ravi and Pachamuthu [101]. The overall results of the PFCHE showed an increase in the heat transfer rate, brake thermal efficiency and heat transfer effectiveness compared to the heat exchanger operating without fins. Ravi et al. [102] applied the PFCHE to internal combustion engines using a water-ethanol mixture as working fluid and achieved improved brake thermal efficiency in the range of 32–39%, and power in the range of 0.35–0.76 kW, when the turbines operated at 1700–4800 rpm, respectively.