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Perspectives for low-temperature waste heat recovery
Energy 176 (2019) 1037e1043
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Energy journal homepage: www.elsevier.com/locate/energy
Perspectives for low-temperature waste heat recovery Z.Y. Xu a, R.Z. Wang a, *, Chun Yang b a b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, 200240, PR China School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 January 2019 Received in revised form 29 March 2019 Accepted 1 April 2019 Available online 2 April 2019
In this forward-looking perspective, the current technologies for low-temperature waste heat recovery are first analyzed from two aspects: (i) the local waste heat recovery technology and (ii) global optimization of energy flow network. Based on the analysis, barriers for the further promotion of waste heat recovery are outlined, and they include the lack of global optimization methodology, distributed waste heat recovery system with high costs, and mismatches between waste heat source and demand. To address these issues, perspectives on three aspects are provided. First, advanced graphical analysis and optimization methodology integrating the heat exchange and energy conversion can promote the userfriendly optimization. Second, concentrated waste heat recovery and supply can save the investment, installation area and operation costs, thereby making the waste heat recovery cost-effective. Third, thermal storage, thermal transportation and high temperature heat pump can better couple the waste heat source and user demand from time-scale, spatial scale and energy grade, respectively. Visions for the future are combined with technical details to provide comprehensive perspectives for the next-step waste heat recovery. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Waste heat Energy conversion Optimization Thermal storage Heat pump
1. Introduction The global economy grows rapidly during past few decades, and it is largely due to industry contributions. Ever-increasing industries need huge amount of power input, giving rise to the problems in both primary energy consumption and CO2 emissions. Moreover, during industry operations, large portion of the energy input is dissipated as waste heat to the ambient in different forms, resulting in severe energy waste. Recovering such waste heat can provide power, heat or cooling output without extra energy input. This increases the energy utilizing efficiency and is considered to be a significant “technology wedge” with the potential to contribute a particular figure for the emission reduction [1]. Considering vast amount of the waste heat, the environmental and economic benefits gained from waste heat recovery would be significant. Recently, many countries begin to pay their attention into this area and they have made many surveys on the industrial waste heat as shown in Table 1. The US Department of Energy did an evaluation on their energy intensive industries including glass, cement, iron/ steel, aluminum, metal casting and ethylene industries. The results
* Corresponding author. 800 Dongchuan Road, Shanghai, PR China. E-mail address: [email protected] (R.Z. Wang). https://doi.org/10.1016/j.energy.2019.04.001 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
showed that about 8.86 EJ energy is being consumed by these industries every year, and it accounts for 9% of the total energy consumption in the US (~92 EJ). Waste heat of 1.56 EJ is dissipated without recovery every year, and low-temperature waste heat below 230 C makes up ~60% of the total waste heat [2]. Similarly, in China the waste heat accounts for 15e40% of the energy input. In typical energy intensive industries in China, waste heat potential of 41 GWTh, 2.9 GW Th and 1.8 GWTh exist in the cement, iron/steel and glass industries, respectively. The capacities of waste heat potential below 150 C are 20 GW Th, 1.9 GWTh and 0.8 GW Th, representing 49%, 66% and 44% of the total waste heat potential, respectively [3]. In the 28 countries in European Union, the total waste heat potential in iron/steel, non-ferrous metal, chemical, non-metallic mineral, food/drink/tobacco, paper/printing and other industries is estimated to be 300 TWh/year, and one third of the waste heat has temperature level below 200 C [4]. The amount of waste heat, especially low-temperature waste heat below 200 C, is vast worldwide, which offers great potential for recovery and reutilization. Compared with the low-temperature waste heat, hightemperature waste heat is more easily to be recovered due to its high energy level which could meet the user demand with different temperature requirements. The high-temperature waste heat could also be used for power generation with relatively mature
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Table 1 Statistics of the waste heat in different regions. Region
Selected industries
Statistics of waste heat
US [2]
Glass, cement, iron/steel, aluminum, metal casting and ethylene industries. Cement, iron/steel, and glass
C C C C C C
China [3] EU [4]
Iron/steel, non-ferrous metal, chemical, food/drink/tobacco, paper/printing and etc.
technologies such as steam turbine or organic Rankine cycle. For the same reason, application of low-temperature waste heat recovery is limited by its temperature level: suitable user demand is not always available, and the heat-power conversion is not efficient for low-temperature waste heat. Besides, as the temperature level of low-temperature waste heat is close to the ambient, heat exchange optimization is crucial to minimize the irreversible loss during the waste heat recovery. All these issues lead to difficulties and challenges in effectively achieving the low-temperature waste heat recovery. To address these issues, the paper first briefly summarizes our options on the low-temperature waste heat recovery. Then, the barriers that prevent from the wide application of lowtemperature waste heat recovery are analyzed. Afterwards, several perspectives are presented on both system optimization and technology improvement. 2. Current options and barriers 2.1. Low-temperature waste heat recovery options Options for waste heat recovery can be classified into direct use and heat conversion. Direct uses of the waste heat include radiation/convection recuperator, passive air preheater, waste heat boiler, economizer, plate heat exchanger and etc. [2]. Heat conversion systems convert the waste heat into heat with different temperatures, cooling or power output. This offer more options for the user side which is important for the waste heat recovery. Options for heat to heat conversion include the vapor compression heat pump, absorption heat pump and absorption heat transformer [5]. Vapor compression heat pump consumes electricity input, while absorption heat pump and absorption heat transformer are driven by heat input. Options for heat to cooling conversion include the absorption chiller and adsorption chiller, and options for heat to power conversion include the organic Rankine cycle [6] and Kalina cycle [7]. Typically, the Kalina cycle is more efficient under higher driven temperature, while the organic Rankine cycle is proper for lower driven temperature [8]. Other heat to power conversion choices include the thermoelectric generator [9], electrochemical systems [10], thermogalvanic cells [11] and pyroelectric energy conversion [12], which are still not ready for large scale and commercial applications. Due to the low temperature level, limited options could be applied for low-temperature waste heat recovery, and direct use for domestic heating is still one of the most commonly used options. However, the domestic heating demand has limited capacity and requires district heating network between the waste heat source and the residential regions. The waste heat conversion technologies still need to be further explored. Except for the waste heat recovery conversion, global heat exchange optimization from the system level [13], industrial zone level [14] to city level [15] is also an important aspect to make full use of the low-temperature waste heat and minimize the irreversible loss. Pinch method is one of the most commonly used method considering both the minimization of irreversible loss and
1.56 EJ waste heat dissipated per year, ~60% of the waste heat is below 230 C. 41 GW, 2.9 GW and 1.8 GW waste heat from the selected industries respectively, 20 GW, 1.9 GW and 0.8 GW of the waste heat is below 150 C. 300 TWh/year of waste heat dissipation, ~1/3 of the waste heat is below 200 C.
practical heat exchange area [16]. 2.2. Barriers of low-temperature waste heat recovery The recovery of low-temperature waste heat is usually complicated. It is affected by the user demand, limited space for heat recovery facilities, economic payback period, and etc. Besides, there are many choices for waste heat recovery and conversion. For instance, the available absorption chillers include the single effect absorption chiller, double effect absorption chiller and double lift absorption chiller with either LiBr-water solution or waterammonia solution [17]. Each option has its own features including working temperature range and maximum temperature lift. This makes it hard to choose the best technical option. Targeting at higher waste heat recovery efficiency, better economic performance and stronger practical feasibility, we highlight the following barriers that prevent from wide applications of lowtemperature waste heat recovery. (1) There is lack of methodology for the heat exchange network optimization when heat-work conversion is concerned. (2) Distributed waste heat recovery increases the installation space requirement, initial investment and operation costs. (3) Mismatches between the waste heat source and the user demand on time, space and energy grade limit the potential of waste heat recovery. The detailed reasons why we propose these barriers and the corresponding perspectives will be presented in next section. 3. Future perspectives 3.1. User-friendly global optimization: advanced graphical analysis with heat conversion Global optimization for industrial waste heat recovery has been studied by many researchers. One of the most effective methods is the pinch method which can analyze the energy flow and optimize the heat exchange network in a straightforward way [16]. Even if some of the waste heat recovery optimization does not follow the exact methodology of pinch method, the optimization is still done by considering an economic minimum temperature difference during heat exchange [18], which shares the same principle as the pinch method. However, the pinch method also has its own limitations. First, it only describes the heat exchange process but fails to include the work input and work output. Second, it is only effective for the heat exchange process without heat conversion, while the situation with both heat exchange and heat conversion is common for low-temperature waste heat recovery [19]. In recent years, many researchers noticed these limitations and proposed various methodologies regarding the combined heat-work conversion and heat exchange network [20], combined work and heat exchange network [21], combined pinch and exergy analysis [22] and so on.
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The optimization needs for different waste heat recovery systems vary from case to case, but currently there is no commonly used method when the heat exchange, heat-work conversion and heatheat conversion are integrated. This makes the optimization timeconsuming, and the development of new methods with the following features is needed: (i) like the pinch method, it should be easily used and expressed; (ii) be able to handle the situation with heat-work and heat-heat conversion systems. Therefore, the following researches are necessary to be done. The first task is to build a new evaluating system, preferably in graphical expression with proper coordinates, allowing to simultaneously evaluate the heat exchange process, work input/output processes and minimum heat exchange temperature difference. Currently, various graphical methods are available including temperature-heat load (T-Q) diagram [13], Carnot factor-heat load (h-Q) diagram [13], temperature-enthalpy (T-H) diagram [23], energy level-heat load diagram [24] and so on. However, these diagrams cannot cover the evaluation of all the mentioned targets. Such a new diagram should cover two aspects including the grade of energy flow and the amount of energy flow. Candidates to evaluate the grade of energy flow include temperature and Carnot factor. Carnot factor takes advantage in the description of the energy grade for both temperature and work, but it fails to include the pinch point. In this case, we think a combination between temperature and work input/output direction would be a compromise. Candidates to evaluate the amount of energy flow include the heat load, enthalpy and exergy. Compared to enthalpy and exergy, heat load is more often and straightforward to be used in the heat exchange network optimization, especially in the waste heat recovery where thermal energy is the major target. However, heat load fails to evaluate the work input and output. If we further consider the evaluating system combining both temperature and work input/ output direction, heat load could correspond to the temperature and work load could correspond to the work input/output direction. Fig. 1 shows an example of such evaluating system. The second task is to make modular diagram of typical systems for low grade heat utilization. For the easy application of evaluating system, the modular diagram should emphasis on its conversion features during its exchange with external heat sources and work
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sources. A possible classification would include the dual-heatsource cycles derived from Carnot cycle/reverse Carnot cycle and triple-heat-source cycles derived from the combination of two dual-heat-source cycles. The real implementations of dual-heatsource cycle include the organic Rankine cycle, the Kalina cycle and the vapor compression cycle. The real implementations of triple-heat-source cycle include the sorption refrigeration cycle, the sorption heat pump cycle and the sorption heat transformer. Fig. 1 shows an example for an integration of the modular compression heat pump into our proposed graphical analyzing diagram. 3.2. Efficient and cost-effective implementation: centralized waste heat recovery and supply Waste heat usually is available in different sections even in one industrial process, which makes the amount and temperature of the waste heat different in these sections. This imposes two barriers for efficient and cost-effective waste heat recovery. Firstly, the recovery of distributed waste heat sources requires small-scale setups which are typically less efficient and more expensive than the large-scale setups. Taking the absorption chiller as an example, its weight, volume, power consumption and cost per unit cooling capacity decrease rapidly with increasing cooling capacity. According to the data from Shuangliang Eco-Energy Co. Ltd. [25], the installation volumes/shipping weights/rated electrical power capacities of hot water fired absorption chiller are 13.05 m3/ 5.80 t/3.15 kW and 96.15 m3/34.70 t/10.45 kW for chillers with cooling capacities of 350 kW and 4650 kW, respectively. If we project the installation volume/shipping weight/rated electrical power capacity of the 350 kW chiller using the cooling capacity ratio of 13.29 (4650 kW/350 kW), the results of 173.43 m3/77.08 t/ 41.90 kW will be obtained. The chiller with cooling capacity of 4650 kW only needs 55.44%, 45.02% and 24.94% of the calculated installation volume/shipping weight/rated electrical power capacity. Hence, such large chiller has the following advantageous features: (1) Smaller volume/cooling capacity ratio indicates smaller surface area/cooling capacity ratio, thereby reducing the loss through the chiller shell to the ambient and increasing its thermal efficiency. (2) Smaller weight/cooling capacity ratio indicates less
T Heat source Irreversible loss
Tmin
Heat -Work
Heat sink
Work - Heat
Heat source Work
Heat load, Q Work output, Wo Work input, Wi
Fig. 1. Example of advanced graphical analysis for waste heat recovery system. Modular expressions are used for heat conversion systems including the heat to work conversion system and work to heat conversion system.
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plant material is needed for the same amount of output, which reduces the initial investment. (3) Smaller electrical power capacity/cooling capacity ratio indicates less electrical consumption during the chiller operation, which is beneficial for both the overall chiller efficiency and the operation cost. Secondly, expect for the source side, the user demand side for waste heat recovery could also be distributed and complicated. This would make the network between the source side and the user side quite complicated, if we want to make full use of the waste heat. Considering the aforementioned issues, it would be more efficient and cost saving if we can upgrade the distributed waste heat recovery system into the centralized waste heat recovery system like an energy hub. Fig. 2 shows our preliminary planning for the centralized waste heat recovery system. First, we can collect the thermal energy from waste heat sources with similar temperature levels and gather it into one stream. Then, the gathered waste heat could be converted into either cooling, heating or power output with large-scale setups. Finally, the useful output could be delivered to wherever they are needed. By applying such centralized waste heat recovery energy hub, part of the energy grade will be lost during the waste heat gathering process, but the complicity and investment of the whole system will be reduced. 3.3. Matching between source and demand In waste heat recovery, capacity of user demand determines the amount of recoverable waste heat, so matching between the waste heat sources and user demands is very important. Expansion of user demand can bring larger capacity for waste heat recovery. However, there is mismatching between the source and demand both in time, and spatial dimensions, and thermal storage/transportation will be necessary to maintain stable input and sufficient output. 3.3.1. Time matching by thermal storage Since waste heat is surplus energy from industrial processes, its capacity and temperature level can vary significantly under different working conditions. However, the waste heat conversion systems typically need heat sources with stable temperature, and the capacity of user demand is not stable. To maintain stable and efficient waste heat conversion and utilization, the three aspects including the source, conversion and user should match with each other. Considering the user demand cannot be easily affected by the
system design, it is better to manipulate the waste heat source and heat conversion system. In the solar thermal systems, thermal storage with water [26] and molten salt [27] has been widely used to overcome the instability and intermittency of solar power. For the same reason, thermal storage could also be used in waste heat recovery system [28] as shown in Fig. 3, but the differences between waste heat recovery and solar thermal utilization also makes their requirements for thermal storage different: (a) both waste heat and solar thermal power are not stable, but waste heat is usually less intermittent than the solar thermal power; (b) the temperature and capacity of solar thermal power is daily and seasonally dependent, but this usually is not the case for the waste heat source. However, since the ambient temperature and user demand change with season, it brings seasonal fluctuations to the energy grade of waste heat. Typical thermal storage for solar energy utilization could be classified into sensible thermal storage, latent thermal storage, sorption thermal storage and chemical reaction thermal storage, based on the energy storage mechanism [29]. Sometimes, the sorption thermal storage and chemical reaction thermal storage are referred as thermal-chemical thermal storage together. Among these different thermal storage mechanisms, the energy storage density and period increase from sensible thermal storage, latent thermal storage, to the thermal-chemical thermal storage, but the cost and complicity also increase at the same time. The sensible thermal storage and latent thermal storage is more proper for short-term storage of waste heat targeting at the instability, while thermal-chemical thermal storage could be used to deal with the energy grade fluctuations and the source-demand matching fluctuations in different seasons. Currently, short-term storage of waste heat with sensible thermal storage [28] and phase change material [30] is more commonly used, and long-term storage of waste heat still needs to be explored in the future. In spite of the high energy storage density from phase change material, water tank is still the most commonly used thermal storage in the waste heat recovery system for two reasons. First, the low cost of water makes it suitable for large scale application. Second, water, as a fluid, has excellent thermal-physical properties which could act as the heat transfer medium and thermal storage medium simultaneously. Currently, most of the latent thermal storage systems employ solid-liquid phase change due to its large latent heat and small volume change during the phase change, this also makes it difficult to extract the waste heat directly. In this case,
Fig. 2. Upgrade from the distributed waste heat recovery “C1-C4” to the centralized waste heat recovery “C”. Waste heat sources, heat conversion systems and user demands are referred as “S1-S4”, “C-C4” and “D1-D4”, respectively. The investment, installation area and efficiency of “C” are better than those of the combination of “C1-C4”.
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Fig. 3. Time-dependent temperature and load profiles of thermal energy flows in the different processes of waste heat utilization including heat source/ heat storage (a), heat storage/ heat conversion (b), and heat conversion/ heat demand (c). The energy flow from waste heat source is adjusted by the heat storage, and then converted to match the user demand.
water or oil could act as the heat transformation fluid from waste heat sources to thermal storage systems. Conversely, the extra heat exchange process between the heat transformation fluid and the waste heat source will reduce the temperature of the stored thermal energy stored. Worse, the presence of additional heat exchange process between the thermal storage system and the thermal transportation fluid to the user will further decrease the temperature of the thermal energy when it reaches the user. As is shown in Fig. 4, assuming temperature differences of 5e10 C in each heat exchange process, the temperature of waste heat will be reduced by 10e20 C. This temperature decrease is fatal for the recovery of waste heat with temperature below 100 C. The ideal Carnot efficiency between 100 C heat source and 30 C heat sink is 18.8%, and this efficiency will decrease to 14.2% with a degrading percentage of 25.5%, when the heat source temperature decreases to 80 C. Minimizing the temperature difference or eliminating the extra heat exchange process could significantly improve the waste heat recovery. Except for enhancing the heat transfer performance of thermal storage system, it is also a good option to explore more fluid-fluid phase change material which simultaneously serves as heat transfer medium and thermal storage medium.
3.3.2. Spatial matching by thermal transportation Nowadays, to fulfill efficient management, supply and logistic chains, safety and environment needs, modern industry plants are arranged together in a centralized zone, where the waste heat is
Fig. 4. Temperature decrease due to extra heat exchanges with thermal storage system. Ts and △T represent the heat source temperature and temperature decrease in one heat exchange process, respectively. System with solid thermal storage (a) has two more heat exchange processes than that with liquid thermal storage (b) as liquid could also serve as the heat transfer fluid.
produced intensively and enormously. The local industrial process and workshop however has limited demand, and thus might not consume all the recovered waste heat. On the other hand, the residential and commercial buildings can be good option to consume the recovered waste heat in the form of space heating, domestic hot water and even space cooling, but these buildings are typically far away from the industrial zone. In order to make full use of such waste heat, it is necessary to connect the concentrated waste heat sources with the far away user demand as shown in Fig. 5. Hence, thermal transportation over long distance can be an effective solution. Long-distance thermal transportation through hot water or steam for district heating has already been used for decades in power plants with combined heat and power generation. The thermal transportation through steam is classified as the first generation of district heating, which requires heavy cement duct for the steam transportation [31]. Such thermal transportation is limited by the heat supply network and has been phased out by the thermal transportation via hot water [31]. Since thermal energy is transported in the form of sensible heat, there are several drawbacks of long-distance thermal transportation via hot water including the high transportation temperature, need of insulation and low energy transportation rate. In order to enhance the energy transportation rate, some researchers proposed the thermal transportation with large temperature difference and absorption plants [32]. Similar to the thermal storage, thermal transportation could also adopt different forms including the latent heat transportation [30]. The difference is that liquid working medium is preferred in longdistance thermal transportation, while both solid and liquid working medium are acceptable for thermal storage. To further enhance the energy transportation rate and reduce the need of insulation, long-distance thermal transportation based on solution absorption phenomenon [33] and chemical reaction [34] has been proposed, but only few experimental results have been reported [35]. The major barrier for the implementation of these technologies include the complicated system, high costs and high/low system pressure. To resolve these issues, researches on absorption or chemical thermal transportation can explore the possibility of open system design and adoption of low cost working pairs. Further studies are required to meet the needs of rapid development of waste heat recovery and smart energy systems. 3.3.3. Energy grade matching by high temperature heat pump According to the statistical data, the amount of industrial waste heat increases dramatically when the waste heat temperature decreases [2]. On the other hand, waste heat with higher temperature could find more application scenarios due to its high energy grade.
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Fig. 5. Thermal transportation for expanding the demand of waste heat recovery (a) and the evolution from thermal storage to (b).
Fig. 6 show the simplified Capacity-Temperature profiles of the waste heat supply and user demand. The overlap between two profiles indicates the potential for waste heat recovery. When the temperature of the waste heat is lifted by the heat pump, the heat supply profile moves rightwards and thus the overlap between supply and demand will be expanded, indicating bigger potential in waste heat recovery. As is shown in Fig. 6, the expanded overlap mainly falls into the high temperature region, suggesting that heat pump with high
Fig. 6. Simplified temperature (T)-load (Q) profiles of waste heat source and user demand. The overlap beneath the source and demand represents the waste heat recovery potential. When the temperature of source is shifted by heat pump, the waste heat recovery potential could be increased, and the increased part is mainly in the high temperature region.
output temperature is helpful to move the boundary forward. However, conventional heat pumps are difficult to reach high output temperature due to the limitation of working media. (1) Traditional working medium like R134a for vapor compression heat pump cannot work efficiently for large temperature lift and has high working pressure with condensation temperature higher than 100 C. Recently, realizing the importance of high temperature heat pump, many researchers have studied the vapor compression heat pump with R600 [36], HFO [37] and water [38] for high temperature application. The corresponding compressor, heat exchange and controlling technologies all need to be further researched with the update of working medium. (2) Absorption heat pump and absorption heat transformer for high temperature operation also need to be investigated as the supplement of vapor compression heat pump since they can be driven by thermal energy directly. However, there are also limitations for the traditional working pairs. Under high temperature, Water-LiBr working pair is corrosive, and ammonia-water working pair is not stable. Recently, ionic liquid is proposed for as the absorbent of absorption working pairs and has potential to solve the corrosive and unstable issues [39], but it also suffers from high cost and high viscosity [40]. More researches about ionic liquid based working pair need to be de done on both working pair improvement and heat/mass transfer enhancement. In addition to the matching of waste heat with pre-existing industry or domestic demand mentioned above, it is also necessary to connect the waste heat recovery with future demand. Considering the energy-saving and emission-reducing capability of waste heat recovery, it can be combined with renewable energy systems [41] and negative emission technologies [42] so as to achieve multiple benefits.
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4. Conclusions Low-temperature waste heat recovery has significant importance to energy saving and emission reduction. At the current stage, low-temperature waste heat recovery faces three major problems: (1) lack of global optimization methodology which can integrate the heat exchange and energy conversion simultaneously; (2) distributed waste heat recovery needs high investment; (3) the mismatches between the waste heat supply and demand in time, space and energy grade limit the potential of waste heat recovery. Perspectives to address these three problems are: (1) Advanced graphical analysis for integrating the heat exchange and energy conversion is needed for the user-friendly optimization. Simplified modular diagram of energy conversion system can be used to focus the analysis on the global optimization. (2) The installation area, investment and operation cost per unit capacity of waste heat recovery plants decrease when the plant capacity. Concentrated waste heat recovery and supply can fully utilize such a property, and hence make the system cost-effective. (3) The mismatches between the waste heat source and user side in time, space and energy grade can be solved by using thermal storage, thermal transportation and high temperature heat pump. Finally, we hope the present comprehensive analysis from the background, current options, and barriers to the future perspectives in different levels would be helpful for researchers who are trying to solve the energy and environmental issues through waste heat recovery.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Perspectives for low-temperature waste heat recovery Z.Y. Xu a, R.Z. Wang a, *, Chun Yang b a b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, 200240, PR China School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 January 2019 Received in revised form 29 March 2019 Accepted 1 April 2019 Available online 2 April 2019
In this forward-looking perspective, the current technologies for low-temperature waste heat recovery are first analyzed from two aspects: (i) the local waste heat recovery technology and (ii) global optimization of energy flow network. Based on the analysis, barriers for the further promotion of waste heat recovery are outlined, and they include the lack of global optimization methodology, distributed waste heat recovery system with high costs, and mismatches between waste heat source and demand. To address these issues, perspectives on three aspects are provided. First, advanced graphical analysis and optimization methodology integrating the heat exchange and energy conversion can promote the userfriendly optimization. Second, concentrated waste heat recovery and supply can save the investment, installation area and operation costs, thereby making the waste heat recovery cost-effective. Third, thermal storage, thermal transportation and high temperature heat pump can better couple the waste heat source and user demand from time-scale, spatial scale and energy grade, respectively. Visions for the future are combined with technical details to provide comprehensive perspectives for the next-step waste heat recovery. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Waste heat Energy conversion Optimization Thermal storage Heat pump
1. Introduction The global economy grows rapidly during past few decades, and it is largely due to industry contributions. Ever-increasing industries need huge amount of power input, giving rise to the problems in both primary energy consumption and CO2 emissions. Moreover, during industry operations, large portion of the energy input is dissipated as waste heat to the ambient in different forms, resulting in severe energy waste. Recovering such waste heat can provide power, heat or cooling output without extra energy input. This increases the energy utilizing efficiency and is considered to be a significant “technology wedge” with the potential to contribute a particular figure for the emission reduction [1]. Considering vast amount of the waste heat, the environmental and economic benefits gained from waste heat recovery would be significant. Recently, many countries begin to pay their attention into this area and they have made many surveys on the industrial waste heat as shown in Table 1. The US Department of Energy did an evaluation on their energy intensive industries including glass, cement, iron/ steel, aluminum, metal casting and ethylene industries. The results
* Corresponding author. 800 Dongchuan Road, Shanghai, PR China. E-mail address: [email protected] (R.Z. Wang). https://doi.org/10.1016/j.energy.2019.04.001 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
showed that about 8.86 EJ energy is being consumed by these industries every year, and it accounts for 9% of the total energy consumption in the US (~92 EJ). Waste heat of 1.56 EJ is dissipated without recovery every year, and low-temperature waste heat below 230 C makes up ~60% of the total waste heat [2]. Similarly, in China the waste heat accounts for 15e40% of the energy input. In typical energy intensive industries in China, waste heat potential of 41 GWTh, 2.9 GW Th and 1.8 GWTh exist in the cement, iron/steel and glass industries, respectively. The capacities of waste heat potential below 150 C are 20 GW Th, 1.9 GWTh and 0.8 GW Th, representing 49%, 66% and 44% of the total waste heat potential, respectively [3]. In the 28 countries in European Union, the total waste heat potential in iron/steel, non-ferrous metal, chemical, non-metallic mineral, food/drink/tobacco, paper/printing and other industries is estimated to be 300 TWh/year, and one third of the waste heat has temperature level below 200 C [4]. The amount of waste heat, especially low-temperature waste heat below 200 C, is vast worldwide, which offers great potential for recovery and reutilization. Compared with the low-temperature waste heat, hightemperature waste heat is more easily to be recovered due to its high energy level which could meet the user demand with different temperature requirements. The high-temperature waste heat could also be used for power generation with relatively mature
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Table 1 Statistics of the waste heat in different regions. Region
Selected industries
Statistics of waste heat
US [2]
Glass, cement, iron/steel, aluminum, metal casting and ethylene industries. Cement, iron/steel, and glass
C C C C C C
China [3] EU [4]
Iron/steel, non-ferrous metal, chemical, food/drink/tobacco, paper/printing and etc.
technologies such as steam turbine or organic Rankine cycle. For the same reason, application of low-temperature waste heat recovery is limited by its temperature level: suitable user demand is not always available, and the heat-power conversion is not efficient for low-temperature waste heat. Besides, as the temperature level of low-temperature waste heat is close to the ambient, heat exchange optimization is crucial to minimize the irreversible loss during the waste heat recovery. All these issues lead to difficulties and challenges in effectively achieving the low-temperature waste heat recovery. To address these issues, the paper first briefly summarizes our options on the low-temperature waste heat recovery. Then, the barriers that prevent from the wide application of lowtemperature waste heat recovery are analyzed. Afterwards, several perspectives are presented on both system optimization and technology improvement. 2. Current options and barriers 2.1. Low-temperature waste heat recovery options Options for waste heat recovery can be classified into direct use and heat conversion. Direct uses of the waste heat include radiation/convection recuperator, passive air preheater, waste heat boiler, economizer, plate heat exchanger and etc. [2]. Heat conversion systems convert the waste heat into heat with different temperatures, cooling or power output. This offer more options for the user side which is important for the waste heat recovery. Options for heat to heat conversion include the vapor compression heat pump, absorption heat pump and absorption heat transformer [5]. Vapor compression heat pump consumes electricity input, while absorption heat pump and absorption heat transformer are driven by heat input. Options for heat to cooling conversion include the absorption chiller and adsorption chiller, and options for heat to power conversion include the organic Rankine cycle [6] and Kalina cycle [7]. Typically, the Kalina cycle is more efficient under higher driven temperature, while the organic Rankine cycle is proper for lower driven temperature [8]. Other heat to power conversion choices include the thermoelectric generator [9], electrochemical systems [10], thermogalvanic cells [11] and pyroelectric energy conversion [12], which are still not ready for large scale and commercial applications. Due to the low temperature level, limited options could be applied for low-temperature waste heat recovery, and direct use for domestic heating is still one of the most commonly used options. However, the domestic heating demand has limited capacity and requires district heating network between the waste heat source and the residential regions. The waste heat conversion technologies still need to be further explored. Except for the waste heat recovery conversion, global heat exchange optimization from the system level [13], industrial zone level [14] to city level [15] is also an important aspect to make full use of the low-temperature waste heat and minimize the irreversible loss. Pinch method is one of the most commonly used method considering both the minimization of irreversible loss and
1.56 EJ waste heat dissipated per year, ~60% of the waste heat is below 230 C. 41 GW, 2.9 GW and 1.8 GW waste heat from the selected industries respectively, 20 GW, 1.9 GW and 0.8 GW of the waste heat is below 150 C. 300 TWh/year of waste heat dissipation, ~1/3 of the waste heat is below 200 C.
practical heat exchange area [16]. 2.2. Barriers of low-temperature waste heat recovery The recovery of low-temperature waste heat is usually complicated. It is affected by the user demand, limited space for heat recovery facilities, economic payback period, and etc. Besides, there are many choices for waste heat recovery and conversion. For instance, the available absorption chillers include the single effect absorption chiller, double effect absorption chiller and double lift absorption chiller with either LiBr-water solution or waterammonia solution [17]. Each option has its own features including working temperature range and maximum temperature lift. This makes it hard to choose the best technical option. Targeting at higher waste heat recovery efficiency, better economic performance and stronger practical feasibility, we highlight the following barriers that prevent from wide applications of lowtemperature waste heat recovery. (1) There is lack of methodology for the heat exchange network optimization when heat-work conversion is concerned. (2) Distributed waste heat recovery increases the installation space requirement, initial investment and operation costs. (3) Mismatches between the waste heat source and the user demand on time, space and energy grade limit the potential of waste heat recovery. The detailed reasons why we propose these barriers and the corresponding perspectives will be presented in next section. 3. Future perspectives 3.1. User-friendly global optimization: advanced graphical analysis with heat conversion Global optimization for industrial waste heat recovery has been studied by many researchers. One of the most effective methods is the pinch method which can analyze the energy flow and optimize the heat exchange network in a straightforward way [16]. Even if some of the waste heat recovery optimization does not follow the exact methodology of pinch method, the optimization is still done by considering an economic minimum temperature difference during heat exchange [18], which shares the same principle as the pinch method. However, the pinch method also has its own limitations. First, it only describes the heat exchange process but fails to include the work input and work output. Second, it is only effective for the heat exchange process without heat conversion, while the situation with both heat exchange and heat conversion is common for low-temperature waste heat recovery [19]. In recent years, many researchers noticed these limitations and proposed various methodologies regarding the combined heat-work conversion and heat exchange network [20], combined work and heat exchange network [21], combined pinch and exergy analysis [22] and so on.
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The optimization needs for different waste heat recovery systems vary from case to case, but currently there is no commonly used method when the heat exchange, heat-work conversion and heatheat conversion are integrated. This makes the optimization timeconsuming, and the development of new methods with the following features is needed: (i) like the pinch method, it should be easily used and expressed; (ii) be able to handle the situation with heat-work and heat-heat conversion systems. Therefore, the following researches are necessary to be done. The first task is to build a new evaluating system, preferably in graphical expression with proper coordinates, allowing to simultaneously evaluate the heat exchange process, work input/output processes and minimum heat exchange temperature difference. Currently, various graphical methods are available including temperature-heat load (T-Q) diagram [13], Carnot factor-heat load (h-Q) diagram [13], temperature-enthalpy (T-H) diagram [23], energy level-heat load diagram [24] and so on. However, these diagrams cannot cover the evaluation of all the mentioned targets. Such a new diagram should cover two aspects including the grade of energy flow and the amount of energy flow. Candidates to evaluate the grade of energy flow include temperature and Carnot factor. Carnot factor takes advantage in the description of the energy grade for both temperature and work, but it fails to include the pinch point. In this case, we think a combination between temperature and work input/output direction would be a compromise. Candidates to evaluate the amount of energy flow include the heat load, enthalpy and exergy. Compared to enthalpy and exergy, heat load is more often and straightforward to be used in the heat exchange network optimization, especially in the waste heat recovery where thermal energy is the major target. However, heat load fails to evaluate the work input and output. If we further consider the evaluating system combining both temperature and work input/ output direction, heat load could correspond to the temperature and work load could correspond to the work input/output direction. Fig. 1 shows an example of such evaluating system. The second task is to make modular diagram of typical systems for low grade heat utilization. For the easy application of evaluating system, the modular diagram should emphasis on its conversion features during its exchange with external heat sources and work
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sources. A possible classification would include the dual-heatsource cycles derived from Carnot cycle/reverse Carnot cycle and triple-heat-source cycles derived from the combination of two dual-heat-source cycles. The real implementations of dual-heatsource cycle include the organic Rankine cycle, the Kalina cycle and the vapor compression cycle. The real implementations of triple-heat-source cycle include the sorption refrigeration cycle, the sorption heat pump cycle and the sorption heat transformer. Fig. 1 shows an example for an integration of the modular compression heat pump into our proposed graphical analyzing diagram. 3.2. Efficient and cost-effective implementation: centralized waste heat recovery and supply Waste heat usually is available in different sections even in one industrial process, which makes the amount and temperature of the waste heat different in these sections. This imposes two barriers for efficient and cost-effective waste heat recovery. Firstly, the recovery of distributed waste heat sources requires small-scale setups which are typically less efficient and more expensive than the large-scale setups. Taking the absorption chiller as an example, its weight, volume, power consumption and cost per unit cooling capacity decrease rapidly with increasing cooling capacity. According to the data from Shuangliang Eco-Energy Co. Ltd. [25], the installation volumes/shipping weights/rated electrical power capacities of hot water fired absorption chiller are 13.05 m3/ 5.80 t/3.15 kW and 96.15 m3/34.70 t/10.45 kW for chillers with cooling capacities of 350 kW and 4650 kW, respectively. If we project the installation volume/shipping weight/rated electrical power capacity of the 350 kW chiller using the cooling capacity ratio of 13.29 (4650 kW/350 kW), the results of 173.43 m3/77.08 t/ 41.90 kW will be obtained. The chiller with cooling capacity of 4650 kW only needs 55.44%, 45.02% and 24.94% of the calculated installation volume/shipping weight/rated electrical power capacity. Hence, such large chiller has the following advantageous features: (1) Smaller volume/cooling capacity ratio indicates smaller surface area/cooling capacity ratio, thereby reducing the loss through the chiller shell to the ambient and increasing its thermal efficiency. (2) Smaller weight/cooling capacity ratio indicates less
T Heat source Irreversible loss
Tmin
Heat -Work
Heat sink
Work - Heat
Heat source Work
Heat load, Q Work output, Wo Work input, Wi
Fig. 1. Example of advanced graphical analysis for waste heat recovery system. Modular expressions are used for heat conversion systems including the heat to work conversion system and work to heat conversion system.
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plant material is needed for the same amount of output, which reduces the initial investment. (3) Smaller electrical power capacity/cooling capacity ratio indicates less electrical consumption during the chiller operation, which is beneficial for both the overall chiller efficiency and the operation cost. Secondly, expect for the source side, the user demand side for waste heat recovery could also be distributed and complicated. This would make the network between the source side and the user side quite complicated, if we want to make full use of the waste heat. Considering the aforementioned issues, it would be more efficient and cost saving if we can upgrade the distributed waste heat recovery system into the centralized waste heat recovery system like an energy hub. Fig. 2 shows our preliminary planning for the centralized waste heat recovery system. First, we can collect the thermal energy from waste heat sources with similar temperature levels and gather it into one stream. Then, the gathered waste heat could be converted into either cooling, heating or power output with large-scale setups. Finally, the useful output could be delivered to wherever they are needed. By applying such centralized waste heat recovery energy hub, part of the energy grade will be lost during the waste heat gathering process, but the complicity and investment of the whole system will be reduced. 3.3. Matching between source and demand In waste heat recovery, capacity of user demand determines the amount of recoverable waste heat, so matching between the waste heat sources and user demands is very important. Expansion of user demand can bring larger capacity for waste heat recovery. However, there is mismatching between the source and demand both in time, and spatial dimensions, and thermal storage/transportation will be necessary to maintain stable input and sufficient output. 3.3.1. Time matching by thermal storage Since waste heat is surplus energy from industrial processes, its capacity and temperature level can vary significantly under different working conditions. However, the waste heat conversion systems typically need heat sources with stable temperature, and the capacity of user demand is not stable. To maintain stable and efficient waste heat conversion and utilization, the three aspects including the source, conversion and user should match with each other. Considering the user demand cannot be easily affected by the
system design, it is better to manipulate the waste heat source and heat conversion system. In the solar thermal systems, thermal storage with water [26] and molten salt [27] has been widely used to overcome the instability and intermittency of solar power. For the same reason, thermal storage could also be used in waste heat recovery system [28] as shown in Fig. 3, but the differences between waste heat recovery and solar thermal utilization also makes their requirements for thermal storage different: (a) both waste heat and solar thermal power are not stable, but waste heat is usually less intermittent than the solar thermal power; (b) the temperature and capacity of solar thermal power is daily and seasonally dependent, but this usually is not the case for the waste heat source. However, since the ambient temperature and user demand change with season, it brings seasonal fluctuations to the energy grade of waste heat. Typical thermal storage for solar energy utilization could be classified into sensible thermal storage, latent thermal storage, sorption thermal storage and chemical reaction thermal storage, based on the energy storage mechanism [29]. Sometimes, the sorption thermal storage and chemical reaction thermal storage are referred as thermal-chemical thermal storage together. Among these different thermal storage mechanisms, the energy storage density and period increase from sensible thermal storage, latent thermal storage, to the thermal-chemical thermal storage, but the cost and complicity also increase at the same time. The sensible thermal storage and latent thermal storage is more proper for short-term storage of waste heat targeting at the instability, while thermal-chemical thermal storage could be used to deal with the energy grade fluctuations and the source-demand matching fluctuations in different seasons. Currently, short-term storage of waste heat with sensible thermal storage [28] and phase change material [30] is more commonly used, and long-term storage of waste heat still needs to be explored in the future. In spite of the high energy storage density from phase change material, water tank is still the most commonly used thermal storage in the waste heat recovery system for two reasons. First, the low cost of water makes it suitable for large scale application. Second, water, as a fluid, has excellent thermal-physical properties which could act as the heat transfer medium and thermal storage medium simultaneously. Currently, most of the latent thermal storage systems employ solid-liquid phase change due to its large latent heat and small volume change during the phase change, this also makes it difficult to extract the waste heat directly. In this case,
Fig. 2. Upgrade from the distributed waste heat recovery “C1-C4” to the centralized waste heat recovery “C”. Waste heat sources, heat conversion systems and user demands are referred as “S1-S4”, “C-C4” and “D1-D4”, respectively. The investment, installation area and efficiency of “C” are better than those of the combination of “C1-C4”.
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Fig. 3. Time-dependent temperature and load profiles of thermal energy flows in the different processes of waste heat utilization including heat source/ heat storage (a), heat storage/ heat conversion (b), and heat conversion/ heat demand (c). The energy flow from waste heat source is adjusted by the heat storage, and then converted to match the user demand.
water or oil could act as the heat transformation fluid from waste heat sources to thermal storage systems. Conversely, the extra heat exchange process between the heat transformation fluid and the waste heat source will reduce the temperature of the stored thermal energy stored. Worse, the presence of additional heat exchange process between the thermal storage system and the thermal transportation fluid to the user will further decrease the temperature of the thermal energy when it reaches the user. As is shown in Fig. 4, assuming temperature differences of 5e10 C in each heat exchange process, the temperature of waste heat will be reduced by 10e20 C. This temperature decrease is fatal for the recovery of waste heat with temperature below 100 C. The ideal Carnot efficiency between 100 C heat source and 30 C heat sink is 18.8%, and this efficiency will decrease to 14.2% with a degrading percentage of 25.5%, when the heat source temperature decreases to 80 C. Minimizing the temperature difference or eliminating the extra heat exchange process could significantly improve the waste heat recovery. Except for enhancing the heat transfer performance of thermal storage system, it is also a good option to explore more fluid-fluid phase change material which simultaneously serves as heat transfer medium and thermal storage medium.
3.3.2. Spatial matching by thermal transportation Nowadays, to fulfill efficient management, supply and logistic chains, safety and environment needs, modern industry plants are arranged together in a centralized zone, where the waste heat is
Fig. 4. Temperature decrease due to extra heat exchanges with thermal storage system. Ts and △T represent the heat source temperature and temperature decrease in one heat exchange process, respectively. System with solid thermal storage (a) has two more heat exchange processes than that with liquid thermal storage (b) as liquid could also serve as the heat transfer fluid.
produced intensively and enormously. The local industrial process and workshop however has limited demand, and thus might not consume all the recovered waste heat. On the other hand, the residential and commercial buildings can be good option to consume the recovered waste heat in the form of space heating, domestic hot water and even space cooling, but these buildings are typically far away from the industrial zone. In order to make full use of such waste heat, it is necessary to connect the concentrated waste heat sources with the far away user demand as shown in Fig. 5. Hence, thermal transportation over long distance can be an effective solution. Long-distance thermal transportation through hot water or steam for district heating has already been used for decades in power plants with combined heat and power generation. The thermal transportation through steam is classified as the first generation of district heating, which requires heavy cement duct for the steam transportation [31]. Such thermal transportation is limited by the heat supply network and has been phased out by the thermal transportation via hot water [31]. Since thermal energy is transported in the form of sensible heat, there are several drawbacks of long-distance thermal transportation via hot water including the high transportation temperature, need of insulation and low energy transportation rate. In order to enhance the energy transportation rate, some researchers proposed the thermal transportation with large temperature difference and absorption plants [32]. Similar to the thermal storage, thermal transportation could also adopt different forms including the latent heat transportation [30]. The difference is that liquid working medium is preferred in longdistance thermal transportation, while both solid and liquid working medium are acceptable for thermal storage. To further enhance the energy transportation rate and reduce the need of insulation, long-distance thermal transportation based on solution absorption phenomenon [33] and chemical reaction [34] has been proposed, but only few experimental results have been reported [35]. The major barrier for the implementation of these technologies include the complicated system, high costs and high/low system pressure. To resolve these issues, researches on absorption or chemical thermal transportation can explore the possibility of open system design and adoption of low cost working pairs. Further studies are required to meet the needs of rapid development of waste heat recovery and smart energy systems. 3.3.3. Energy grade matching by high temperature heat pump According to the statistical data, the amount of industrial waste heat increases dramatically when the waste heat temperature decreases [2]. On the other hand, waste heat with higher temperature could find more application scenarios due to its high energy grade.
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Fig. 5. Thermal transportation for expanding the demand of waste heat recovery (a) and the evolution from thermal storage to (b).
Fig. 6 show the simplified Capacity-Temperature profiles of the waste heat supply and user demand. The overlap between two profiles indicates the potential for waste heat recovery. When the temperature of the waste heat is lifted by the heat pump, the heat supply profile moves rightwards and thus the overlap between supply and demand will be expanded, indicating bigger potential in waste heat recovery. As is shown in Fig. 6, the expanded overlap mainly falls into the high temperature region, suggesting that heat pump with high
Fig. 6. Simplified temperature (T)-load (Q) profiles of waste heat source and user demand. The overlap beneath the source and demand represents the waste heat recovery potential. When the temperature of source is shifted by heat pump, the waste heat recovery potential could be increased, and the increased part is mainly in the high temperature region.
output temperature is helpful to move the boundary forward. However, conventional heat pumps are difficult to reach high output temperature due to the limitation of working media. (1) Traditional working medium like R134a for vapor compression heat pump cannot work efficiently for large temperature lift and has high working pressure with condensation temperature higher than 100 C. Recently, realizing the importance of high temperature heat pump, many researchers have studied the vapor compression heat pump with R600 [36], HFO [37] and water [38] for high temperature application. The corresponding compressor, heat exchange and controlling technologies all need to be further researched with the update of working medium. (2) Absorption heat pump and absorption heat transformer for high temperature operation also need to be investigated as the supplement of vapor compression heat pump since they can be driven by thermal energy directly. However, there are also limitations for the traditional working pairs. Under high temperature, Water-LiBr working pair is corrosive, and ammonia-water working pair is not stable. Recently, ionic liquid is proposed for as the absorbent of absorption working pairs and has potential to solve the corrosive and unstable issues [39], but it also suffers from high cost and high viscosity [40]. More researches about ionic liquid based working pair need to be de done on both working pair improvement and heat/mass transfer enhancement. In addition to the matching of waste heat with pre-existing industry or domestic demand mentioned above, it is also necessary to connect the waste heat recovery with future demand. Considering the energy-saving and emission-reducing capability of waste heat recovery, it can be combined with renewable energy systems [41] and negative emission technologies [42] so as to achieve multiple benefits.
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4. Conclusions Low-temperature waste heat recovery has significant importance to energy saving and emission reduction. At the current stage, low-temperature waste heat recovery faces three major problems: (1) lack of global optimization methodology which can integrate the heat exchange and energy conversion simultaneously; (2) distributed waste heat recovery needs high investment; (3) the mismatches between the waste heat supply and demand in time, space and energy grade limit the potential of waste heat recovery. Perspectives to address these three problems are: (1) Advanced graphical analysis for integrating the heat exchange and energy conversion is needed for the user-friendly optimization. Simplified modular diagram of energy conversion system can be used to focus the analysis on the global optimization. (2) The installation area, investment and operation cost per unit capacity of waste heat recovery plants decrease when the plant capacity. Concentrated waste heat recovery and supply can fully utilize such a property, and hence make the system cost-effective. (3) The mismatches between the waste heat source and user side in time, space and energy grade can be solved by using thermal storage, thermal transportation and high temperature heat pump. Finally, we hope the present comprehensive analysis from the background, current options, and barriers to the future perspectives in different levels would be helpful for researchers who are trying to solve the energy and environmental issues through waste heat recovery.
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