1.3.4.1 Industrial Waste Heat Recovery
Supercritical CO2 heat pumps have many application opportunities such in medium-sized indus-tries (such as food processing) and institutional buildings (such as hospitals), where process and/ or domestic hot water is required at temperatures above 60°C for process, cleaning, washing or
Two-stage compressor
Outdoor coil Indoor coil
Vapor injection line
Internal heat exchanger
Injection EXV
EXV EXV
Four-way reversing valve
FIGURE 1.26 Schematic drawn of a cold climate air-to-air heat pump with inverter-driven two-stage compressor and vapor injection with internal heat exchanger in the heating mode only. (From http://www . mitsubishielectric.ca/en/hvac/residential.html, accessed May 15, 2015.)
domestic usages, and residential sector. In industry, these devices can simultaneously provide heat-ing (i.e., hot water or air) and coolheat-ing (i.e., cold water or air), thus maximizheat-ing the systems’ overall efficiency and consuming up to 27% less electricity than two separate systems operating under identical thermal conditions (Byrne et al. 2009). In many industrial facilities, low-enthalpy waste heat in liquid or air forms is often available at temperatures of below 45°C, whereas the cold water to be heated comes from municipal water networks at relatively low temperatures. To efficiently recover of low-temperature waste heat from industrial processes effluents, a two-stage heat recov-ery systems, including a preheating heat exchanger and a CO2 supercritical heat pump as the first and second stages, respectively, can be successfully implemented (Figure 1.27) (IEA 2015a; Minea 2015). In such a system, if the waste heat fluid enters the heat recovery system at temperatures between 15°C and 45°C, it can be cooled down to between 10°C and 38°C, depending on the actual inlet temperature of the cold water. In cold climates, it generally varies from a minimum of 7°C (in the winter) to a maximum of 17°C (in the summer). By bypassing (or not) the preheating heat exchanger, the cold city water can be heated up to 38°C before entering the supercritical CO2 heat pump. The process is controlled by the regulating valve RV according to the relative magnitude of the inlet temperatures of the heat source and heat sink thermal carriers.
An industrial-scale supercritical CO2 heat pump has been implemented in a Canadian dairy plant (Minea 2014c). It includes a 25 kW compressor (nominal power input) and is integrated between two of the plant’s industrial processes (Figure 1.28a). It recovers energy from one pro-cess at temperatures below 0°C and provides heat to another process at temperatures above 85°C.
Because of the low critical temperature of CO2, these boundary thermal conditions allow the heat pump to operate according to a supercritical cycle with evaporation at subcritical pressures and heat rejection at pressures above the CO2 critical pressure (7.377 MPa-a). So, unlike conventional subcritical heat pump cycles, heat is not supplied by means of refrigerant condensation, but by
coolerGas Evaporator
Waste heat (water) source
IHX
Pump
CO2 compressor Low-pressure
side receiver
EXV
P F
F Waste out
Waste in
Hot water out Cold water inPR
RV HEX
Cold water Water
storage tanks To consumers
FIGURE 1.27 Two-stage laboratory-scale heat recovery system with supercritical CO2 heat pump. EXV, expansion valve; F, flow meter; HEX, preheating heat exchanger; IHE, internal heat exchanger; PR, pressure regulator; RV, 3-way regulating valve. (From Minea, V., High-temperature heat pump-assisted softwood dryer:
Sizing and control requirements & energy performances, in 24th International Congress of Refrigeration (ICR2015), August 16–22, Yokohama, Japan, 2015; IEA, Industrial energy-related systems and technologies Annex 13, IEA Heat Pump Program Annex 35, Application of Industrial Heat Pumps, final report, Part 1, 2015a; reprinted with permission from IEA HPT Annex 13/35 operating agent.)
Gas cooler
Evaporator
P1 Compressor
EXV
Storage tank
P2
F F
Cold water inlet
Heat source storage tank
12
34 Hot water (a)(b)Absolute pressure (MPa)
Mass enthalpy (kJ/kg)
−2.7°C
109°C
137°C 1
2 3 4
Tevap, avrg = −9°C 2.73
11.32Gas cooler Evaporator
27.6°C 2s ds = 0 −9°C
7.377C
Receiver
FIGURE 1.28 (a) Schematic diagram of the industrial-scale supercritical CO2 heat pump implemented in a Canadian dairy plant. (From Minea, V., Efficient process integration and cooling & heating energy performance of supercritical CO2 heat pumps, in 11th IEA Heat Pump Conference 2014, May 12–16, Montréal, Québec, Canada, 2014c; reprinted with permission from IEA HPP Centre.) (b) Example of a typical (average) thermodynamic cycle. C, critical point; P1, hot brine pump; P2, cold water pump; EXV, expansion valve; F, flow meter.
cooling compressed high-pressure CO2 inside a special heat exchanger called gas cooler. As can be seen in Figure 1.28a, IHEX is included in the heat pump thermodynamic cycle. Because both the cooling and heating thermal effects of the unit can be used by the industrial facility, the overall system energy efficiency is further improved. In this system, the superheated refrigerant (state 1) enters the compressor where it is brought to higher pressures and temperatures (state 2) by a nonisentropic compression process (1–2) (Note: the theoretical isentropic process is shown as 1-2s) (Figure 1.28b). The discharge pressure is controlled based on the varying amount of refrigerant inside the high-pressure side of the system and not by the saturation pressure as is the case with conventional HFC refrigerants. By reducing the valve opening, the CO2 mass flow rate decreases, the CO2 accumulates in the high-pressure side and its pressure thus rises. Conversely, by increasing the valve opening, the high-side pressure decreases, whereas the excess charge is stored as a liquid inside the low-pressure side of the refrigeration circuit. This particular control strategy requires special algorithms to adjust the adiabatic expansion process in order to keep the evaporation almost constant and optimize the compressor discharge pressure and the gas cooler outlet temperature.
Two 79.9 kW (heating capacity) supercritical CO2 heat pumps each equipped with 24-kW CO2 reciprocating compressors for simultaneous hot and cold water production have been imple-mented in Japan in a frozen noodle (10,000 tons/year) industrial process. In Japan, food process-ing consists of multiple processes at different temperatures such as cleanprocess-ing, sterilization, boilprocess-ing, cooling, freezing, and drying and, traditionally, gas burners and/or heavy oil-fired steam boilers are used as heating sources and chillers are used as cooling sources (IEA 2015b). The boiling process of the frozen noodle processing requires hot water at temperatures higher than 80°C, tradition-ally supplied by steam boilers. The supercritical CO2 heat pumps have been introduced between the boiling and the cooling processes in order to simultaneously produce hot and cold water. The process includes three steam boilers (3500 kg/h total) and two boiling pools (3000 L) where the water is reheated near the boiling temperature (98°C) with steam from boilers. The plant operates about 16 h/day during about 250 days/year. Hot water (90°C) produced by the heat pump flows through a heat exchanger and is stored in a hot water tank during the night and delivered at higher temperature to the boiling pools in the morning. This allows the steam boilers to start up later and thus reduce the peak load of steam boilers. About 45 m3/day of stored hot water is daily delivered from the storage tank to consumers. The rest of stored hot water is used to preheat the boilers’ feed water, which contributes to lower the thermal load of steam boilers. The cold water supplied by the heat pump at 7°C is used to further cool the water in the raw water tank for the cooling pools, thus reducing the load of water chillers. Additional cooling (up to 3°C) is provided by the existing water chillers. By absorbing heat from the cold side (water) and rejecting heat to the hot side (water) of the industrial process, these simultaneous hot and cold water producing heat pumps are able to heat water from 17°C to 85°C (3000 L/h) for sterilization and boiling processes, and cool water from 10°C to 5°C (5000 L/h), with heat pump heating and cooling COPs of 3.0, and 2.1, respectively, and overall simultaneous heating and cooling COP of 5.1. The hot water supply to the boiling pools also reduced the heating load of the steam boilers, resulting in about 4% CO2 emissions reduction (IEA 2015b). Anstett (2006) reported the performance of an air-source supercritical CO2 heat pump water heater installed in a hospital to deliver hot water at temperatures between 60°C and 80°C with water inlet temperatures as low as 10°C and ambient air (as the heat source) temperatures from
−20°C to 40°C. At ambient temperature of −5°C the heat pump delivered hot water at 70°C with a COP of 2.5. In the case of low-energy houses located in moderate and cold climates, Stene (2005, 2007) proposed to apply supercritical CO2 heat pumps to supply multiple, simultaneous or alternate, space and hot water heating loads. In this concept, the gas cooler was partitioned to separately sup-ply domestic water preheating, space heating, and domestic water reheating. Hihara (2006) also reported that supercritical CO2 heat pumps can provide combined domestic hot water and hydronic floor heating with seasonal COPs of 2.7, value that includes motor efficiency and transient thermal losses from the hot water storage tank.
1.3.4.2 Industrial Drying
A supercritical CO2 heat pump has been implemented in Japan as an air heater for a drying process (Figure 1.29) (IEA 2015b). In order to reduce the fuel consumption for drying as well as the drying time, hot air at around 120°C is frequently used for drying in industrial casing painting of electrical transformers (about 35,000 units per year). To generate such a drying hot air, conventional boilers, burners, or electric heaters are usually used. In traditional painting processes, air circulated in the drying ovens, after the electrodeposition process, is heated up to about 170°C by liquid propane gas burners. After the top coating process, the air circulated in the ovens is heated up to about 155°C.
Partial ventilation is necessary to prevent the circulated air from contamination, which causes a decrease in the thermal efficiency of the facilities. In addition, exclusive chillers are used for keeping the temperature of the electrocoating baths at 29°C. Because heat pumps using conventional HFC refrigerants can heat air up to maximum 70°C, supercritical CO2 heat pumps have been employed to generate hot air at temperatures higher than 80°C. Hence, a 110 kW (heating capacity) supercritical CO2 heat pump equipped with a semi-hermetic, reciprocating, inverter controlled capacity (30–65 Hz) CO2 compressor, has been installed in order to preheat the fresh air and, thus, simultaneously cooling (35 kW cooling capacity) the electrocoating baths (Figure 1.29). In this system, outdoor fresh air is first heated up to between 80°C and 120°C by the supercritical CO2 heat pump by recovering waste heat at temperatures varying from −9°C to 35°C. Then, the air is heated further up to required temperatures by the liquid propane gas burners prior entering the air drying ovens. The preheating operation with
Steam
Drying oven Top coat Drying oven
Degrease Chemical conversion
Hot water cleaning
Exhaust gas
LPG LPG
Outdoor air
Heat pump air heater Chiller
Compressed air Air compressor
Waste heat recovery heat exchanger Cold water storage tank Electrocoating
FIGURE 1.29 System flow diagram of drying process in painting application in Japan. (From IEA, Industrial energy-related systems and technologies Annex 13, IEA Heat Pump Program Annex 35, Application of Industrial Heat Pumps, Final Report, Part 2, 2015b; redrawn and reprinted with permission from IEA HPT Annex 13/35 operating agent.)
supercritical CO2 heat pump reduces the liquid propane gas burners’ heating load. CO2 heat pump may also recover heat from the cold water stored at 15°C. After being cooled in the evaporator, cold water at 10°C returns to the cold water storage tank. This heat recovery process reduces the thermal load of the water chiller. If the heat recovered from the cold water is not sufficient, the heat pump will recover more heat from the air compressor discharge. This system has reduced the process average natural gas consumption by 24.1%, the running costs by 12% and the CO2 greenhouse gas emissions by 13.1%.
Drum dryers have been used for a long time in homes for drying of laundries (see Section 1.2.2.2). They are household appliances used to remove moisture from clothes and other textiles.
Because traditional refrigerants, as HFC-134a (GWP factor of 1300), have negative environmen-tal impacts, the academia and industry started looking at supercritical CO2 heat pumps as drum dryers about 15 years ago. Schmidt et al. (1998) compared the thermodynamic behavior of two dehumidification heat pump cycles: a subcritical process with R-134a and a supercritical process with CO2. Their simulation results showed that both cycles are equivalent in terms of energy efficiency. However, better compression efficiency was expected with CO2, as well as improved energy efficiency. Klöcker et al. (2001) studied the feasibility of using CO2 as a working fluid for laundry heat pump dryers and compared it with R-134a systems. They found that the former did not need more energy than the latter, and that the CO2 supercritical cycle was suitable for heat pump dryers due to the good environmental characteristics and thermal properties of CO2. These authors compared the energy performance of two optimized CO2 heat pumps with those of a conventional electrically heated dryer and concluded that if the drying time is not the key requirement, energy savings of about 65% can be achieved. In addition, if a high water extraction rate is sought, energy savings of about 53% can be provided, including fan energy consumption, but with much shorter drying times than that of the conventional system. Klöcker et al. (2002) built a laboratory prototype laundry dryer using CO2 as a working fluid. Their experiments showed that the heat pumps used in laundry dryers at 50°C–60°C exhibited a significant energy savings potential (between 53% and 65%) compared to conventional high temperature (130°C) drying methods. However, their drying cycles with supercritical CO2 heat pumps in batch mode were relatively short (about 54 min). The reported experimental results showed that the end of the drying cycles was dependent on the maximum air temperature entering the evaporator (40°C) rather than the material final MC. These authors also showed that up to 35% of each dry-ing cycle was a transient process with air temperatures leavdry-ing the gas cooler (enterdry-ing the drum) being below the desired set points (50°C–60°C). Honma et al. (2008) presented an experimental study on a compact heat pump dryer using CO2 as a refrigerant for domestic cloth washers/dry-ers. The authors mentioned the requirement to balance the amount of heat provided by the air with the heat supplied by the gas cooler and to adapt the refrigerant cycle based on the progress of the drying cycle. They controlled the EXV to keep the superheat constant in order to main-tain the targeted heating capacity at a constant level of 2.7 kW and reduce the drying time. The gas cooler had a heat transfer surface about 40% higher than that of the evaporator for the same airflow rate, and the heat pump COP was estimated at 4.07. The tests performed revealed that the prototype was able to reduce electric power consumption by 59.2% and the drying time by 52.5% in comparison with electrical heater drying systems. Furthermore, the authors estimated that the drying time can be further reduced by 3% by keeping the refrigerant superheating tem-perature at 6°C–10°C.