THEORETICAL COMPARISONS BETWEEN ABSORPTION HEAT PUMP AND ELECTRICAL HEAT PUMP FOR LOW TEMPERATURE HEATING

Transcription

1 ID:191 HEOREICAL COMPARISONS BEWEEN ABSORPION HEA PUMP AND ELECRICAL HEA PUMP FOR LOW EMPERAURE HEAING Wei WU, Wenxing SHI, Baolong WANG, Xianting LI (*) (*) Department of Building Science, singhua University, Beijing, , China ABSRAC Energy consumption for and DHW is very high, and heat pump technologies are becoming popular. Air source absorption heat pump (ASAHP) and air source electrical heat pump (), ground source absorption heat pump (GSAHP) and ground source electrical heat pump () are theoretically compared. Comparisons between ASAHP and indicated that: (1) ASAHP is advantageous over under colder conditions; (2) exergy efficiency of ASAHP is higher than that of in most conditions; (3) high-efficiency ASAHP is always more emission-reduction than, while lowefficiency ASAHP is more emission-reduction than when ambient is below 0 C. Comparisons between GSAHP and revealed that: (1) annual primary energy efficiency of high-efficiency GSAHP is higher than that of, while low-efficiency GSAHP is worse than ; (2) GSAHP is more suitable for colder regions with reduced thermal imbalance; (3) borehole number of can be reduced by 40~50% using GSAHP. 1. INRODUCION he building energy consumption occupies 30 40% of the total energy consumption in developed countries, and contributes to 15 25% in developing countries (UBESRC, 2013). he energy consumption related to and domestic hot water (DHW) can reach 40 60% for residential buildings and 20 60% for commercial buildings in developed countries (Westphalen and Koszalinski, 2001). In developing countries like China, and DHW in urban areas take up more than 27% of the total building energy consumption (Li et al., 2015). Most of the space systems in developed countries are based on direct use of fossil fuel and electricity in both commercial and residential buildings (Westphalen and Koszalinski, 2001; Mahapatra and Leif, 2008). Most residential DHW heaters are also based on fossil fuels or electric, with nearly 40% homes in the USA using electric heater, due to their easy installation and operation (Hepbasli and Yildiz, 2009). In developing countries like China, coal boiler and combined heat and power (CHP) are the most widely used for space in northern China; while for DHW in urban areas, electric heater and gas heater are the most widely used (UBESRC, 2013). Direct use of fossil fuels is of low energy efficiency and serious air pollution. he average heat efficiency of coal boiler is about 70%, while that of gas boiler is about 90%. Besides, the fuel burning will emit a substantial amount of pollutants, such as CO 2, SO 2, NO X and particulate matters (Wang et al., 2011; Wu et al., 2013a). Although the electric hot water systems produce few emissions in the user side, the primary energy efficiency is as low as 33% (UBESRC, 2013). here is an urgent necessity to improve the energy efficiency and reduce the emissions of the conventional systems. Recently, air source electrical heat pump () is extensively used for and DHW for its energy saving and simple system configuration (Cabrol and Rowley, 2012). But the performance and reliability become worse when the air temperature is low (Ma et al., 2003). Besides, the ground source electrical heat pump () is also becoming popular due to the advantages of energy saving and the incentives from the government (Sarbu and Sebarchievici, 2014; i et al., 2003). However, large-scale applications of will cause several problems: serious pressure on electricity generation and transmission; underground thermal imbalance and soil temperature drop (You et al., 2014; Geng et al., 2013); large amount of boreholes and occupied lands. Apart from the electrical heat pump, the absorption heat pump also shows great energy saving potential in heat supply (Wu et al., 2014a; Li et al., 2011). he air source absorption heat pump (ASAHP) and ground source absorption heat pump (GSAHP) had been proposed to solve the above problems (Li et al., 2012; Wu

2 et al., 2012; Wu et al., 2014b). he previous studies revealed that ASAHP was advantageous in cold climate, and GSAHP was potential in reducing the thermal imbalance (Wu et al., 2013b; Wu et al., 2014c; Wu et al., 2014d). he objective of this work is to theoretically compare the difference between absorption heat pump and electrical heat pump, in terms of energy efficiency, exergy efficiency, CO 2 emission, thermal imbalance and borehole number, to investigate the applicability of different heat pump systems in different occasions. 2. MEHODOLOGY 2.1. System description he currently widely used and are taken as the baselines, based on which the ASAHP and GSAHP will be compared, respectively. he schematics of air source systems are specifically shown in Figure 1, including the basic, single-effect ASAHP and GAX-cycle (generator absorber heat exchange) ASAHP. he basic uses R22 as the refrigerant due to its common applications at present and high efficiency of the system. Other more environmentally friendly refrigerants can also be similarly studied in the future. As for the ASAHP, NH 3-LiNO 3 is used for single-effect due to its high efficiency while NH 3-H 2O is used for GAX-cycle to avoid the possible risk of crystallization that may occur in NH 3-salt solutions under higher generation temperatures (Wu et al., 2013c). he single-effect ASAHP is driven by hot stream of 130 C, so it can be combined with the city network or district boiler to greatly enhance the primary energy efficiency (PEE) of the conventional systems. When the natural gas are well available for, then the GAX-cycle ASAHP can be adopt to achieve further higher PEE owing to the inner heat recovery between the absorber and generator (Jawahar and Saravanan, 2010). (a) (b) Single-effect ASAHP (c) GAX-cycle ASAHP Figure 1 Schematics of, single-effect ASAHP and GAX-cycle ASAHP 2.2. Indexes and calculation methods For the air source systems, the energy efficiency, exergy efficiency and emission factor will be comparatively analyzed. he supply hot water temperature for all the systems is set as 45 C in this work. he performance of basic is obtained from the manufacture s performance data, with a lower ambient temperature limit of -10 C. For ambient temperatures below -10 C, the double-stage compression cycle is chosen to suit the severe conditions, and the performance is obtained by simulation. For the ASAHP systems, the performance is mainly obtained based on the mass and energy balance of each component (Herold et al., 1996): m m out in (1) mout xout min xin (2) mout hout minhin (3) UA LMD (4) c p m f ( out in ) (5) where UA is the product of the heat transfer coefficient and the heat transfer area, kw/k; LMD is the logarithmic mean temperature difference, K; is the capacity of each heat exchanger, kw; m in and m out are the inlet and outlet mass flow rate, kg/s; h in and h out are the inlet and outlet enthalpy, kj/kg; x in and x out are the inlet and outlet solution concentration, kg/kg; p flow rate, kg/s; in and out are the inlet and outlet temperature, K. c is the specific heat, kj/(kg K); he coefficient of performance () of ASAHP and in mode is defined as: m f is the mass

3 c ASAHP: g a c ; : (6) W c where c, a and g are the heat exchange rates of condenser, absorber and generator, respectively, kw; W c is the electricity consumption rate of compressor, kw. he primary energy efficiency (PEE) of ASAHP and in mode is defined as: c a c ASAHP: PEE ; : PEE (7) g /boiler W c/power where boiler is the boiler efficiency, which is 70% for coal boiler and 90% for gas boiler; power is the power generation efficiency, which is set as 33%. he exergy (E) of ASAHP and in mode is defined as: ASAHP: E 0 (c a )[1 ln( r2 0 s1 g [1 ln( )] r2 ; : s1 r1 r1 )] E c [1 where 0 is the reference temperature, set as the ambient temperature, K; s1 and r1 are the supply and return temperature in primary side, K; and r2 are the supply and return water temperature in user side, K. he exergy PEE (EPEE) of ASAHP and in mode is defined as: ASAHP: 0 (c a )[1 ln( )] r2 r2 EPEE g 0 (1 ) boiler p ; : EPEE c [1 Wc power 0 W c r2 0 ln( r2 0 (1 ) p ln( )] r2 )] (8) r2 (9) where p is the effective temperature of the primary energy source, which is 973 K for coal and 1773 K for gas based on the current technology level. he CO 2 emission factor of ASAHP and is defined as the CO 2 emission amount divided by the supplied quantity (kg/kwh): PE FCO2 CO2 emission factor (10) s where PE is the consumed primary energy, kg for coal and Nm 3 for gas; FCO2 is the emission factor of primary energy, which is 2.66 kg/kgce for coal and kg/nm 3 for gas; s is the supplied quantity, kwh. he above indexes for air source systems also apply to the ground source systems. In addition, the annual efficiency, thermal imbalance and borehole number will be comparatively analyzed for ground source systems. he annual primary energy efficiency (APEE) of GSAHP and is defined as: ASAHP: APEE heaitng boiler boiler ; : APEE heaitng power where and are the total load and load, kwh; heaitng and are the average and. he thermal imbalance ratio is defined in terms of the total heat rejection into the soil and the total heat extraction from the soil: IR 1 (1 ) 1 max[ (1 ), 1 (1 1 (1 ) 100% )] If IR is close to 0%, the thermal balance is well maintained and the soil temperature is kept stable; if IR is far above 0%, heat accumulation will occur and soil temperature will increase year by year; if IR is obvious lower than 0%, cold accumulation will occur and soil temperature will decrease year by year. he borehole number is designed based on the maximum extraction rate in winter, and the auxiliary device can be integrated when the borehole heat exchanger is insufficient to deliver the heat rejection rate in summer. he borehole number is estimated using the simplified method: power (11) (12)

4 Borehole number 1000q q Etraction,max L length Borehole q Etraction, max is the maximum heat extraction rate in winter, kw; q length is the heat exchange rate per length based on the empirical data, which is set as 30 W/m; L Borehole is the borehole depth, which is 100 m in this work. hough this estimation method is not very accurate, it can indicate the difference of the required borehole numbers between GSAHP and, and it is fair to conduct the comparison using the same criteria. More accurate methods based on software can be used in further analysis. he economic analysis is not focused in this paper, and can be found in the previous work (Wu et al., 2014b; Wu et al., 2014c). 3. COMPARISONS OF AIR SOURCE HEA PUMP 3.1. Energy efficiency For and DHW in different climate zones, the ambient temperature can cover a wide range of -30~40 C. s of (single-stage if ambient>-10 C, double-stage if ambient<-10 C), low-efficiency ASAHP (coal-based single-effect if ambient>-20 C, coal-based double-stage if ambient<-20 C) and highefficiency ASAHP (gas-based GAX-cycle) under different ambient temperatures are shown in Figure 2. Under the condition of 7 C ambient and 45 C hot water, the rated is 3.746, and 1.757, respectively. he of each heat pump increases as the ambient temperature rises, and yields a much higher increasing rate. he varies in the range of 2.077~5.835 for, 1.243~1.627 for lowefficiency ASAHP and 1.461~2.117 for high-efficiency ASAHP, respectively. So has much higher values, especially in the warmer conditions (13) PEE Ambient temperature (C) Figure 2 of and ASAHP under different ambient temperatures Ambient temperature (C) Figure 3 PEE of and ASAHP under different ambient temperatures Considering the energy conversion efficiency of primary energy into the driving power of (electricity) and ASAHP (heat), PEE of different heat pumps are compared in Figure 3. It is found that the difference between ASAHP and has been narrowed since the energy conversion efficiency of heat producing is much higher than that of electricity generation. he PEE varies in the range of 0.685~1.926 for, 0.870~1.139 for low-efficiency ASAHP and 1.315~1.905 for high-efficiency ASAHP, respectively. he high-efficiency ASAHP is better than when the ambient temperature is lower than 33 C, while the low-efficiency ASAHP becomes better than when the ambient temperature is lower than 0 C. hat is to say, ASAHP is much more advantageous under colder conditions in the viewpoint of PEE, though its is not high. he year-round PEE is subject to the temperature and load characteristics of a specific application. For the low-efficiency ASAHP, if the time when ambient temperature below 0 C lasts for a long period, such as space in cold regions, then it can be more energy saving than. As for the high-efficiency ASAHP, since the PEE stays the most competitive in almost the whole covered temperature range, it will be more energy saving than under most circumstances Exergy efficiency Considering the energy grade of heat and electricity, E of different heat pumps are illustrated in Figure 4. It is obvious that E of ASAHP is much higher than that of, the difference increases under lower

5 ambient temperatures. his is because uses high-grade electricity as the driving power, while the ASAHP is driven by heat with lower energy grades. E varies in the range of 77~59 for, 0.726~46 for low-efficiency ASAHP and 0.711~52 for high-efficiency ASAHP, respectively. hough increases as the ambient temperature rises, E declines since the exergy of both input and output decreases as the ambient temperature rises, but the former decreases more rapidly, as indicated in eq. (8). It is nothing that using driving source of lower energy grade contributes to the higher E of low-efficiency ASAHP compared with high-efficiency ASAHP. Considering both the energy grade and primary energy efficiency, EPEE of different heat pumps are presented in Figure 5. Similar to E, the EPEE also decreases as the ambient temperature rises. However, the EPEE of ASAHP is not always higher than that of, with a turning point of 0 C for lowefficiency ASAHP and 12 C for high-efficiency ASAHP. he EPEE varies in the range of 10~22 for, 66~13 for low-efficiency ASAHP and 0.350~18 for high-efficiency ASAHP, respectively. he EPEE difference between high-efficiency ASAHP and is lower than the PEE difference, due to the higher primary energy grade of gas used by high-efficiency ASAHP than coal used by E 0.5 EPEE Ambient temperature (C) Figure 4 E of and ASAHP under different ambient temperatures Ambient temperature (C) Figure 5 EPEE of and ASAHP under different ambient temperatures 3.3. Emission factor he CO 2 emission factors of different heat pumps are demonstrated in Figure 6 to evaluate the impact on environment. he high-efficiency ASAHP is more emission-reduction than in the whole covered conditions, while the low-efficiency ASAHP is more emission-reduction than when the ambient temperature is below 0 C. uantitatively, the CO 2 emission factor varies in the range of 77~0.170 kg/kwh for, 0.376~87 kg/kwh for low-efficiency ASAHP and 0.152~0.105 kg/kwh for highefficiency ASAHP, respectively. 0.5 CO 2 emission factor (kg/kwh) Ambient temperature (C) Figure 6 CO 2 emission factor of and ASAHP under different ambient temperatures 4. COMPARISONS OF GROUND SOURCE HEA PUMP he above analysis results for air source systems also apply to ground source systems, so they will not be repeated in this section. Additionally, the annual efficiency, thermal imbalance and borehole number will be

6 specifically analysed for ground source systems. he average soil temperature and load characteristics have main influences on the focused indexes for comparison Annual efficiency Considering the conversion efficiency of primary energy in both and season, the APEEs under different ratios of total load to total load ( / Cooling ) and different soil temperatures are compared in Figure 7 and Figure 8. Figure 7 reveals that the APEE of GSAHP is more sensitive to the load characteristic, since the and differ more greatly for GSAHP. Besides, higher load contributes to higher APEE of GSAHP due to its high PEE in mode but low PEE in load. When / Cooling increases from 1 to 10, APEE varies in the range of 1.222~1.259 for, ~1.204 for low-efficiency GSAHP and 1.211~1.541 for high-efficiency GSAHP, respectively. Figure 8 implies that APEE of is more sensitive to the soil temperature, resulted by more sensitive of. he APEE of high-efficiency GSAHP is higher than that of in most conditions; while low-efficiency GSAHP is not as good as, but the difference gets smaller under higher loads, such as space in severely cold regions APEE 0.8 APEE / Cooling Figure 7 APEE of and GSAHP under different loads (soil temperature=12 C) soil (C) Figure 8 APEE of and GSAHP under different soil temperatures ( / Cooling = 4) 4.2. hermal imbalance IR (%) 0-20 IR (%) / Cooling Figure 9 IR of and GSAHP under different load characteristics (soil temperature=12 C) soil (C) Figure 10 IR of and GSAHP under different soil temperatures ( / Cooling = 4) o evaluate the thermal imbalance of different ground source systems, the thermal imbalance ratio (IR) under different load characteristics and soil temperatures are demonstrated in Figure 9 and Figure 10. It is obvious in Figure 9 that IR of GSAHP is much more positive than that of, due to its lower and lower, which leads to more heat rejection and less heat extraction. For all of the heat pump systems, IR first decreases (less heat accumulation) and then increases (more cold accumulation) as / Cooling increases. he previous analysis indicated that IR with ±20% caused small soil temperature variation during long-term operations. For, thermal balance can be well kept under / Cooling of 1.5~2.0, with IR values in the range of 13.3~-13.5%. For low-efficiency GSAHP, thermal balance can be

7 well kept under / Cooling of 6.5~9.5, with IR values in the range of 16.4~-18.1%. As for highefficiency GSAHP, thermal balance can be well kept under / Cooling of 3.0~5.0, with IR values in the range of 18.5~-14.1%. If there is no auxiliary or devices, GSAHP is more suitable for colder regions with higher loads, in the viewpoint of soil thermal balance. Figure 10 shows that IR is not sensitive to the soil temperature for a given / Cooling. Under the load characteristic of / Cooling =4, high-efficiency GSAHP maintains good thermal balance, while low-efficiency GSAHP leads to heat accumulation but brings about cold accumulation Borehole number he design borehole number greatly affects the system investment and the occupied area. he required borehole numbers based on mode under different soil temperatures are presented in Figure 11. For all of the systems, the borehole number increases as the soil temperature rises, since the will be higher and more heat can be extracted from the soil at a given load. he borehole number of is more sensitive to the soil temperature because the changes more greatly. Compared with, the borehole number of low-efficiency GSAHP can be reduced by 50%, and that of high-efficiency GSAHP can be reduced by 40%. his is of great significance to the reduction of occupied area as well as the reduction of borehole investment. It should be kept in mind that the borehole number is designed based on the load, and auxiliary devices need to be integrated if the load is much higher. Fortunately, the auxiliary device like tower is far cheaper than the borehole heat exchanger Borehole number soil (C) Figure 11 Required borehole number of and GSAHP under different soil temperatures 5. CONCLUSIONS he energy consumption for and DHW is very high and the conventional heat supply systems are mainly based on direct use of fossil fuels and electricity, which are of low energy efficiency and serious air pollution. Heat pump technologies have great potential in energy saving and emission reduction. Absorption heat pump and electrical heat pump had been comprehensively compared to investigate the applicability in different occasions. Comparisons between ASAHP and indicated that: (1) hough its is not very high, ASAHP is much more advantageous over under colder conditions in the viewpoint of PEE; (2) E of ASAHP is always much higher than that of, and EPEE of ASAHP is higher than that of under colder conditions; (3) is always more emission-reduction than, while low-efficiency ASAHP is more emission-reduction than when the ambient is below 0 C. Comparisons between GSAHP and revealed that: (1) APEE of high-efficiency GSAHP is higher than that of in most conditions, while low-efficiency GSAHP is not as good as ; (2) GSAHP is more suitable for colder regions with higher loads, in the viewpoint of soil thermal balance; (3) Compared with, the borehole number of low-efficiency GSAHP can be reduced by 50%, and that of high-efficiency GSAHP can be reduced by 40%. 6. ACKNOWLEDGEMENS he authors gratefully acknowledge the support from the Natural Science Foundation for Distinguished Young Scholars of China (grant No ).

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