Installation and Operation of a Solar Cooling and Heating System Incorporated with Air-Source Heat Pumps
Round 1
Reviewer 1 Report
An analysis is presented of the annual performance of a solar absorption cooling system with an air-source heat pump backup. The annual average COP and solar fraction are reported.
This is a somewhat brief paper that does not really offer any detailed insightful discussion of the system operation or how it could be improved. Nonetheless it is still of some value as a case study of a functioning solar cooling system, and the results are presented clearly. The paper could be improved significantly by offering some further critical insight such as a quantitative comparison to results from other solar-cooling systems, or to theoretical performance calculations. Some information on the system capital and running costs would also be of value for comparison with other systems. At present the conclusions are more of a summary of the results, and should be developed further to demonstrate the contribution/novelty of this paper more clearly and/or provide directions for future work.
Some more detailed comments as follows:
Introduction line 43: please provide a definition of the “period average COP”.
Introduction line 52: please provide a definition of the COP and the solar COP.
The term “cooling performance factor” is used frequently in the paper. For clarity, is this the same as the coefficient of performance? An equation explicitly showing how this parameter is calculated should be included. The calculation of the solar fraction, SF, should also be shown.
Section 3: A clearer description should be provided of how the system functions in winter. Are the solar collectors and heat pumps both used to provide space heating via the fan coil units?
Line 140: A volume of 4 T is reported for the buffer tank. Should this be 4 m3?
Author Response
Thank you very much for your comments and suggestions. The following major revisions have been made according to your opinions:
1. The English language and style have been improved by MDPI English editing service.
2. Some recent references have been additionally reviewed in the Section 1. Introduction:
• Lines 52-56: S. Rosiek, F.J. Batlles. Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building. Renew Energy, 34 (2009), pp. 1423-1431
• Lines 74-79: O. Marc, F. Sinama, J.-P. Praene, F. Lucas, J. Castaing-Lasvignottes. Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application. Appl Therm Eng, 90 (2015), pp. 980-993
• Lines 79-83: Z.Y. Xu, R.Z. Wang, H.B. Wang. Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system. Int J Refrig, 59 (2015), pp. 135-143
• Lines 69-74: Y. Hang, M. Qu, R. Winston, L. Jiang, B. Widyolar, H. Poiry. Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced. Energy Build, 82 (2014), pp. 746-757
• Lines 101-103: A. Shirazi, R.A. Taylor, G.L. Morrison, S.D. White. Solar-powered absorption chillers: A comprehensive and critical review. Energy Convers Manage, 171 (2018), pp. 59-81
3. A clearer description has been provided of how the system functions in winter and the system performance has been presented in the paper.
Lines 150-154: During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 °C and 60 °C produced by the solar collector field is directly used for building heating. The heat pumps additionally provide hot water when solar energy is insufficient.
Lines 220-235: During the heating period in winter, the valves V1, V2, V7~V10, V13~V18 were open and V3~V6, V11 and V12 were closed. The controlling system was achieved through the following main cycles:
(1) The solar loop
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4≥4 °C; the pump, P1,was switched off when the temperature difference T3-T4＜2 °C.
(2) The heating loop
The pump, P2, was switched on when the lower layer temperature of the buffer tank T15＜55 °C AND the difference between the upper layer temperature of the hot water storage tank and the lower layer temperature of the buffer tank T4-T15≥2 ; The pump, P2, was switched on when the lower layer temperature of the buffer tank T15≥60 °C OR T4≤T15.
(3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T15＜45 °C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T15≥50 °C OR the fan coils were shut down.
Performance analysis:
Figure 8. The monthly average outdoor temperature and building cooling and heating energy demand in 2018.
Figure 15. The monthly supplied cooling/heating energy, the electricity consumption and the coefficient of performance of the heat pumps COPhp in 2018.
Figure 17. The monthly solar fraction SFn and electricity savings in 2018.
4. The definitions and equations of the solar collector efficiency(lines 262-267), coefficient of performance of the absorption chiller COPchiller (lines 286-295), coefficient of performance of the heat pumps COPhp (lines 324-326) and solar fraction SFn (lines 327-339) have been presented in the paper.
Furthermore, an experimental error has been analyzed, including the errors in all measured variables and the description of the instruments used. Please see the above-mentioned equations and Table 2-The instruments and their calibration range and uncertainty.
5. A comparison between the system performance without and with the heat pumps was presented in the paper.
Lines 350-363: In order to determine the influence of the heat pumps on the system performance, the heat pumps were switched off on 25, 27 and 28 July 2018 and the system performance was analyzed. As shown in Figure 16, the energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. The daily global irradiation onto collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWh on 23 July. However, the collector field yield and cooling energy yield were 183.9 kWh and 133.7 kWh respectively, which were only 60% and 50% of those values on 23 July when incorporated with the heat pumps. The solar faction decreased from 0.62 on 23 July to 0.15 on 28 July. The reason was that it took much more time to produce sufficient chilled water by providing the absorption chiller with hot water by using solar energy than by using both solar energy and the heat pumps.
Figure 16. The irradiation onto collector area, collector field yield, cooling energy yield and solar fraction SFn,c on 23, 25, 27 and 28 July, the heat pumps were switched off on 25, 27 and 28 July,2018.
6. The Section 4: Discussion and conclusion has been revised to get a scientific approach to the described problem, please see lines 384-409.
An absorption solar cooling and heating system assisted by two air-source heat pumps located in Ningbo City, China was studied in this paper. The system was run in 2018 and the operation results were evaluated. Based on this study the main conclusions are as follows:
The solar collector field was comprised of 40 all-glass evacuated tube modules with a total aperture area of 120 m2. The annual average collector efficiency was 44% for building cooling and 42% for building heating.
The single-stage and LiBr–water based absorption chiller had a cooling capacity of 35 kW. The monthly average coefficient of performance COPchiller ranged between 0.68 and 0.76 in 2018. The COPchiller increased with the rising hot water inlet temperature.
Two air-source heat pumps each with a rated cooling capacity of 23.8 kW and heating capacity of 33 kW were used as the auxiliary system for the solar cooling and heating installation. The average coefficient of performance COPhp,c was 2.83 in 2018, with minimums of 2.74 and 2.75 in July and August, respectively. The average coefficient of performance for heating COPhp,h was 3.20.
The energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. In comparison with the case combined with the heat pumps under the similar irradiation condition, the collector field yield and cooling energy yield decreased by more than 40%, and the solar faction decreased from 0.62 to 0.15.
Two kinds of the operation modes were conducted, namely the building cooling and heating modes. The annual mean solar fractions for cooling and heating were 56.6% and 62.5%, respectively. The yearly electricity saving was 10,158.6 kWh when combined with the solar cooling and heating system, which accounted for 41.1% of the total electricity consumption for building cooling and heating and corresponds to 5,445 kg of CO2 emissions prevented from being released into the atmosphere.
Introduction line 43: please provide a definition of the “period average COP”.
Please see line 44: average COP during the period of 20 days of monitoring (25/07/2003 to 19/08/2003).
Introduction line 52: please provide a definition of the COP and the solar COP.
Please see Section 1. Introduction:
Line 43: The COP is the ratio of cooling energy yield and heat supplied by the generator.
Line 54: The solar COP is the ratio of cooling energy yield and the total solar energy available.
The term “cooling performance factor” is used frequently in the paper. For clarity, is this the same as the coefficient of performance? An equation explicitly showing how this parameter is calculated should be included. The calculation of the solar fraction, SF, should also be shown.
Yes, it is the same as the coefficient of performance. We have revised all of the terms “cooling performance factor” into “coefficient of performance”. The definitions and equations of the solar collector efficiency(lines 262-267), coefficient of performance of the absorption chiller COPchiller (lines 286-295), coefficient of performance of the heat pumps COPhp (lines 324-326) and solar fraction SFn (lines 327-339) have been presented in the paper.
Section 3: A clearer description should be provided of how the system functions in winter. Are the solar collectors and heat pumps both used to provide space heating via the fan coil units?
Lines 150-154: During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 °C and 60 °C produced by the solar collector field is directly used for building heating. The heat pumps additionally provide hot water when solar energy is insufficient.
Lines 220-235: During the heating period in winter, the valves V1, V2, V7~V10, V13~V18 were open and V3~V6, V11 and V12 were closed. The controlling system was achieved through the following main cycles:
(1) The solar loop
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4≥4 °C; the pump, P1,was switched off when the temperature difference T3-T4＜2 °C.
(2) The heating loop
The pump, P2, was switched on when the lower layer temperature of the buffer tank T15＜55 °C AND the difference between the upper layer temperature of the hot water storage tank and the lower layer temperature of the buffer tank T4-T15≥2 ; The pump, P2, was switched on when the lower layer temperature of the buffer tank T15≥60 °C OR T4≤T15.
(3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T15＜45 °C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T15≥50 °C OR the fan coils were shut down.
Performance analysis:
Figure 8. The monthly average outdoor temperature and building cooling and heating energy demand in 2018.
Figure 15. The monthly supplied cooling/heating energy, the electricity consumption and the coefficient of performance of the heat pumps COPhp in 2018.
Figure 17. The monthly solar fraction SFn and electricity savings in 2018.
Author Response File: 
Author Response.doc
Reviewer 2 Report
The article analyzed must not be published for the following reasons:
- It is a more technical report and the scientific originality of its topic and the innovation of its findings are very poor;
- the nomenclature is absent;
- the paper analyses the cooling performance while the plant is able to satisfy heating and cooling demands;
- the paper shows experimental results but it does not include an experimental error analysis, including the errors in all measured variables and the description of the instruments used;
- usually a the solar cooling plant includes a dry cooler to dissipate the thermal energy when there is no energy demand from the end users. This prevents overheating of the solar collectors;
- the Athors evaluate the yearly electricity savings (and the avoided CO2) but is not clear if they have considered the electric consumption of a solar cooling plant mainly due to auxiliaries (pumps, cooling tower, etc);
- The description of operation modes (3.2) is not complete and clear. For istance:
o in the “Solar loop” the Authors states that “the pump P1 is switched on when…T1 OR T2 > 85°C”. In my opinion it depends from T5 too;
o in the “Cooling loop” the Authors states that “the pump P2 is switched on when…T4 > 80°C”. In my opinion also the existence of a cooling demand from the fan coils has to bee considered;
- The references are very few (11, 2 in Chinese), not recent (2012), and consider only a limited number of types of solar cooling systems (based on adsorption and absorption).
Author Response
Thank you very much for your comments and suggestions. The following major revisions have been made according to your opinions:
1. The English language and style have been improved by MDPI English editing service.
2. Some recent references have been additionally reviewed in the Section 1. Introduction:
• Lines 52-56: S. Rosiek, F.J. Batlles. Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building. Renew Energy, 34 (2009), pp. 1423-1431
• Lines 74-79: O. Marc, F. Sinama, J.-P. Praene, F. Lucas, J. Castaing-Lasvignottes. Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application. Appl Therm Eng, 90 (2015), pp. 980-993
• Lines 79-83: Z.Y. Xu, R.Z. Wang, H.B. Wang. Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system. Int J Refrig, 59 (2015), pp. 135-143
• Lines 69-74: Y. Hang, M. Qu, R. Winston, L. Jiang, B. Widyolar, H. Poiry. Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced. Energy Build, 82 (2014), pp. 746-757
• Lines 101-103: A. Shirazi, R.A. Taylor, G.L. Morrison, S.D. White. Solar-powered absorption chillers: A comprehensive and critical review. Energy Convers Manage, 171 (2018), pp. 59-81
3. A clearer description has been provided of how the system functions in winter and the system performance has been presented in the paper.
Lines 150-154: During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 °C and 60 °C produced by the solar collector field is directly used for building heating. The heat pumps additionally provide hot water when solar energy is insufficient.
Lines 220-235: During the heating period in winter, the valves V1, V2, V7~V10, V13~V18 were open and V3~V6, V11 and V12 were closed. The controlling system was achieved through the following main cycles:
(1) The solar loop
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4≥4 °C; the pump, P1,was switched off when the temperature difference T3-T4＜2 °C.
(2) The heating loop
The pump, P2, was switched on when the lower layer temperature of the buffer tank T15＜55 °C AND the difference between the upper layer temperature of the hot water storage tank and the lower layer temperature of the buffer tank T4-T15≥2 ; The pump, P2, was switched on when the lower layer temperature of the buffer tank T15≥60 °C OR T4≤T15.
(3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T15＜45 °C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T15≥50 °C OR the fan coils were shut down.
Performance analysis:
Figure 8. The monthly average outdoor temperature and building cooling and heating energy demand in 2018.
Figure 15. The monthly supplied cooling/heating energy, the electricity consumption and the coefficient of performance of the heat pumps COPhp in 2018.
Figure 17. The monthly solar fraction SFn and electricity savings in 2018.
4. The definitions and equations of the solar collector efficiency(lines 262-267), coefficient of performance of the absorption chiller COPchiller (lines 286-295), coefficient of performance of the heat pumps COPhp (lines 324-326) and solar fraction SFn (lines 327-339) have been presented in the paper.
Furthermore, an experimental error has been analyzed, including the errors in all measured variables and the description of the instruments used. Please see the above-mentioned equations and Table 2-The instruments and their calibration range and uncertainty.
5. A comparison between the system performance without and with the heat pumps was presented in the paper.
Lines 350-363: In order to determine the influence of the heat pumps on the system performance, the heat pumps were switched off on 25, 27 and 28 July 2018 and the system performance was analyzed. As shown in Figure 16, the energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. The daily global irradiation onto collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWh on 23 July. However, the collector field yield and cooling energy yield were 183.9 kWh and 133.7 kWh respectively, which were only 60% and 50% of those values on 23 July when incorporated with the heat pumps. The solar faction decreased from 0.62 on 23 July to 0.15 on 28 July. The reason was that it took much more time to produce sufficient chilled water by providing the absorption chiller with hot water by using solar energy than by using both solar energy and the heat pumps.
Figure 16. The irradiation onto collector area, collector field yield, cooling energy yield and solar fraction SFn,c on 23, 25, 27 and 28 July, the heat pumps were switched off on 25, 27 and 28 July,2018.
6. The Section 4: Discussion and conclusion has been revised to get a scientific approach to the described problem, please see lines 384-409.
An absorption solar cooling and heating system assisted by two air-source heat pumps located in Ningbo City, China was studied in this paper. The system was run in 2018 and the operation results were evaluated. Based on this study the main conclusions are as follows:
The solar collector field was comprised of 40 all-glass evacuated tube modules with a total aperture area of 120 m2. The annual average collector efficiency was 44% for building cooling and 42% for building heating.
The single-stage and LiBr–water based absorption chiller had a cooling capacity of 35 kW. The monthly average coefficient of performance COPchiller ranged between 0.68 and 0.76 in 2018. The COPchiller increased with the rising hot water inlet temperature.
Two air-source heat pumps each with a rated cooling capacity of 23.8 kW and heating capacity of 33 kW were used as the auxiliary system for the solar cooling and heating installation. The average coefficient of performance COPhp,c was 2.83 in 2018, with minimums of 2.74 and 2.75 in July and August, respectively. The average coefficient of performance for heating COPhp,h was 3.20.
The energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. In comparison with the case combined with the heat pumps under the similar irradiation condition, the collector field yield and cooling energy yield decreased by more than 40%, and the solar faction decreased from 0.62 to 0.15.
Two kinds of the operation modes were conducted, namely the building cooling and heating modes. The annual mean solar fractions for cooling and heating were 56.6% and 62.5%, respectively. The yearly electricity saving was 10,158.6 kWh when combined with the solar cooling and heating system, which accounted for 41.1% of the total electricity consumption for building cooling and heating and corresponds to 5,445 kg of CO2 emissions prevented from being released into the atmosphere.
- It is a more technical report and the scientific originality of its topic and the innovation of its findings are very poor;
The above-mentioned revisions have been made to obtain a scientific approach to the described problem.
- the nomenclature is absent;
Yes, it has been added in lines 416-417.
- the paper analyses the cooling performance while the plant is able to satisfy heating and cooling demands;
A clearer description has been provided of how the system functions in winter and the system performance has been presented in the paper.
Lines 150-154: During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 °C and 60 °C produced by the solar collector field is directly used for building heating. The heat pumps additionally provide hot water when solar energy is insufficient.
Lines 220-235: During the heating period in winter, the valves V1, V2, V7~V10, V13~V18 were open and V3~V6, V11 and V12 were closed. The controlling system was achieved through the following main cycles:
(1) The solar loop
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4≥4 °C; the pump, P1,was switched off when the temperature difference T3-T4＜2 °C.
(2) The heating loop
The pump, P2, was switched on when the lower layer temperature of the buffer tank T15＜55 °C AND the difference between the upper layer temperature of the hot water storage tank and the lower layer temperature of the buffer tank T4-T15≥2 ; The pump, P2, was switched on when the lower layer temperature of the buffer tank T15≥60 °C OR T4≤T15.
(3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T15＜45 °C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T15≥50 °C OR the fan coils were shut down.
Performance analysis:
Figure 8. The monthly average outdoor temperature and building cooling and heating energy demand in 2018.
Figure 15. The monthly supplied cooling/heating energy, the electricity consumption and the coefficient of performance of the heat pumps COPhp in 2018.
Figure 17. The monthly solar fraction SFn and electricity savings in 2018.
- the paper shows experimental results but it does not include an experimental error analysis, including the errors in all measured variables and the description of the instruments used;
The definitions and equations of the solar collector efficiency(lines 262-267), coefficient of performance of the absorption chiller COPchiller (lines 286-295), coefficient of performance of the heat pumps COPhp (lines 324-326) and solar fraction SFn (lines 327-339) have been presented in the paper.
Furthermore, an experimental error has been analyzed, including the errors in all measured variables and the description of the instruments used. Please see the above-mentioned equations and Table 2-The instruments and their calibration range and uncertainty.
- usually a the solar cooling plant includes a dry cooler to dissipate the thermal energy when there is no energy demand from the end users. This prevents overheating of the solar collectors;
We have used an air-cooled radiator to prevent overheating of the solar collectors (lines 143-144). The cooling tower is used to extract waste heat from the chiller (lines 149-150).
- the Authors evaluate the yearly electricity savings (and the avoided CO2) but is not clear if they have considered the electric consumption of a solar cooling plant mainly due to auxiliaries (pumps, cooling tower, etc);
Yes, we have considered the electric consumption of a solar cooling plant mainly due to auxiliaries (pumps, cooling tower, etc).
- The description of operation modes (3.2) is not complete and clear. For instance:
o in the “Solar loop” the Authors states that “the pump P1 is switched on when…T1 OR T2 > 85°C”. In my opinion it depends from T5 too;
o in the “Cooling loop” the Authors states that “the pump P2 is switched on when…T4 > 80°C”. In my opinion also the existence of a cooling demand from the fan coils has to been considered;
Yes, it has been corrected, see lines 208-215:
(1) The solar loop
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4 ≥4 K; the pump, P1, was switched off when the temperature difference T3-T4＜2 K.
(2) The cooling loop
The pump, P2, was switched on when the upper layer temperature of the hot water storage tank T4≥80 °C AND the fan coils were turned on; the pump, P2, was switched off when T4＜70 °C OR the fan coils were shut down.
- The references are very few (11, 2 in Chinese), not recent (2012), and consider only a limited number of types of solar cooling systems (based on adsorption and absorption).
The following recent references have been additionally reviewed in the Section 1. Introduction:
· S. Rosiek, F.J. Batlles. Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building. Renew Energy, 34 (2009), pp. 1423-1431
· O. Marc, F. Sinama, J.-P. Praene, F. Lucas, J. Castaing-Lasvignottes. Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application. Appl Therm Eng, 90 (2015), pp. 980-993
· Z.Y. Xu, R.Z. Wang, H.B. Wang. Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system. Int J Refrig, 59 (2015), pp. 135-143
· Y. Hang, M. Qu, R. Winston, L. Jiang, B. Widyolar, H. Poiry. Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced. Energy Build, 82 (2014), pp. 746-757
· A. Shirazi, R.A. Taylor, G.L. Morrison, S.D. White. Solar-powered absorption chillers: A comprehensive and critical review. Energy Convers Manage, 171 (2018), pp. 59-81
Author Response File: Author Response.doc
Reviewer 3 Report
In this paper, authors introduce a simple and not innovative solar cooling system equipped with two air-source electric heat pumps as back up, installed in Ningbo City, China. They also show performance data recorded during the summer 2018 operation.
Even though the investigated topic is interesting, the system introduced and data showed are not new neither surprising. Furthermore, most of the performance recorded and presented are lower than the current state of art of such kind of systems, and not any innovative feature has been presented. Why are the system showed more interesting or the data introduced more useful to the reader respect to the hundreds of similar data available in the literature and already published (tens of handbooks have been written showing the same data recorded in demo sites all around the world)? All the information provided to the reader are formally correct, however they are extremely simple and seem to be more suited to a technical report than to a paper in a scientific journal. The introduction section is extremely short and the number of references used by the authors is reduced, such demonstrating a not adequate propaedeutic literature review.
Furthermore, authors do not introduce winter operation performance even if the system can be operated during the winter.
Moreover, the following criticisms have been identified in the text:
· Line 158: T13 is the cooling water inlet temperature of the absorption chiller;
· Lines 168-169: Why is P1 operated only when the outlet temperature of the solar field is greater than 80° C? The best way to harvest the most heat from a solar field is to operate the pump when the outlet temperature from the solar collectors is higher (+ few offset degrees) than the temperature of the water into the high temperature tank. Authors should discuss about their choice.
· Figure 8: legend is missing
· All figures: the font should be replaced since the one used makes reading extremely difficult.
Concluding, since the paper is extremely simple and does not deliver new information to the reader, it is not suitable to be published on Energies. The paper is REJECTED.
Author Response
Thank you very much for your comments and suggestions. The following major revisions have been made according to your opinions:
1. The English language and style have been improved by MDPI English editing service.
2. Some recent references have been additionally reviewed in the Section 1. Introduction:
• Lines 52-56: S. Rosiek, F.J. Batlles. Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building. Renew Energy, 34 (2009), pp. 1423-1431
• Lines 74-79: O. Marc, F. Sinama, J.-P. Praene, F. Lucas, J. Castaing-Lasvignottes. Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application. Appl Therm Eng, 90 (2015), pp. 980-993
• Lines 79-83: Z.Y. Xu, R.Z. Wang, H.B. Wang. Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system. Int J Refrig, 59 (2015), pp. 135-143
• Lines 69-74: Y. Hang, M. Qu, R. Winston, L. Jiang, B. Widyolar, H. Poiry. Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced. Energy Build, 82 (2014), pp. 746-757
• Lines 101-103: A. Shirazi, R.A. Taylor, G.L. Morrison, S.D. White. Solar-powered absorption chillers: A comprehensive and critical review. Energy Convers Manage, 171 (2018), pp. 59-81
3. A clearer description has been provided of how the system functions in winter and the system performance has been presented in the paper.
Lines 150-154: During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 °C and 60 °C produced by the solar collector field is directly used for building heating. The heat pumps additionally provide hot water when solar energy is insufficient.
Lines 220-235: During the heating period in winter, the valves V1, V2, V7~V10, V13~V18 were open and V3~V6, V11 and V12 were closed. The controlling system was achieved through the following main cycles:
(1) The solar loop
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4≥4 °C; the pump, P1,was switched off when the temperature difference T3-T4＜2 °C.
(2) The heating loop
The pump, P2, was switched on when the lower layer temperature of the buffer tank T15＜55 °C AND the difference between the upper layer temperature of the hot water storage tank and the lower layer temperature of the buffer tank T4-T15≥2 ; The pump, P2, was switched on when the lower layer temperature of the buffer tank T15≥60 °C OR T4≤T15.
(3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T15＜45 °C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T15≥50 °C OR the fan coils were shut down.
Performance analysis:
Figure 8. The monthly average outdoor temperature and building cooling and heating energy demand in 2018.
Figure 15. The monthly supplied cooling/heating energy, the electricity consumption and the coefficient of performance of the heat pumps COPhp in 2018.
Figure 17. The monthly solar fraction SFn and electricity savings in 2018.
4. The definitions and equations of the solar collector efficiency(lines 262-267), coefficient of performance of the absorption chiller COPchiller (lines 286-295), coefficient of performance of the heat pumps COPhp (lines 324-326) and solar fraction SFn (lines 327-339) have been presented in the paper.
Furthermore, an experimental error has been analyzed, including the errors in all measured variables and the description of the instruments used. Please see the above-mentioned equations and Table 2-The instruments and their calibration range and uncertainty.
5. A comparison between the system performance without and with the heat pumps was presented in the paper.
Lines 350-363: In order to determine the influence of the heat pumps on the system performance, the heat pumps were switched off on 25, 27 and 28 July 2018 and the system performance was analyzed. As shown in Figure 16, the energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. The daily global irradiation onto collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWh on 23 July. However, the collector field yield and cooling energy yield were 183.9 kWh and 133.7 kWh respectively, which were only 60% and 50% of those values on 23 July when incorporated with the heat pumps. The solar faction decreased from 0.62 on 23 July to 0.15 on 28 July. The reason was that it took much more time to produce sufficient chilled water by providing the absorption chiller with hot water by using solar energy than by using both solar energy and the heat pumps.
Figure 16. The irradiation onto collector area, collector field yield, cooling energy yield and solar fraction SFn,c on 23, 25, 27 and 28 July, the heat pumps were switched off on 25, 27 and 28 July,2018.
6. The Section 4: Discussion and conclusion has been revised to get a scientific approach to the described problem, please see lines 384-409.
An absorption solar cooling and heating system assisted by two air-source heat pumps located in Ningbo City, China was studied in this paper. The system was run in 2018 and the operation results were evaluated. Based on this study the main conclusions are as follows:
The solar collector field was comprised of 40 all-glass evacuated tube modules with a total aperture area of 120 m2. The annual average collector efficiency was 44% for building cooling and 42% for building heating.
The single-stage and LiBr–water based absorption chiller had a cooling capacity of 35 kW. The monthly average coefficient of performance COPchiller ranged between 0.68 and 0.76 in 2018. The COPchiller increased with the rising hot water inlet temperature.
Two air-source heat pumps each with a rated cooling capacity of 23.8 kW and heating capacity of 33 kW were used as the auxiliary system for the solar cooling and heating installation. The average coefficient of performance COPhp,c was 2.83 in 2018, with minimums of 2.74 and 2.75 in July and August, respectively. The average coefficient of performance for heating COPhp,h was 3.20.
The energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. In comparison with the case combined with the heat pumps under the similar irradiation condition, the collector field yield and cooling energy yield decreased by more than 40%, and the solar faction decreased from 0.62 to 0.15.
Two kinds of the operation modes were conducted, namely the building cooling and heating modes. The annual mean solar fractions for cooling and heating were 56.6% and 62.5%, respectively. The yearly electricity saving was 10,158.6 kWh when combined with the solar cooling and heating system, which accounted for 41.1% of the total electricity consumption for building cooling and heating and corresponds to 5,445 kg of CO2 emissions prevented from being released into the atmosphere.
· Line 158: T13 is the cooling water inlet temperature of the absorption chiller;
Yes, it has been corrected.
· Lines 168-169: Why is P1 operated only when the outlet temperature of the solar field is greater than 80° C? The best way to harvest the most heat from a solar field is to operate the pump when the outlet temperature from the solar collectors is higher (+ few offset degrees) than the temperature of the water into the high temperature tank. Authors should discuss about their choice.
Yes, it has been corrected, see lines 208-211:
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4 ≥4 K; the pump, P1, was switched off when the temperature difference T3-T4＜2 K.
· Figure 8: legend is missing
The legend has been added into the figure 9 (it is Figure 9 after the revision) .
· All figures: the font should be replaced since the one used makes reading extremely difficult.
Yes, the front of all figures has been corrected to be shown clearly.
Author Response File: Author Response.doc
Reviewer 4 Report
This study conducted experimental research about a solar absorption cooling system assisted by heat pumps. Overall, the paper is concise and shows many graphs for easy understanding about operating of the system. However, this work probably needs to be revised, therefore I recommend that this paper cannot be accepted in the present form. Some specific comments are hereunder.
1. The work lacks originality, since many research work has been done on solar absorption cooling system. The authors of this work have not even attempted to compare their results with other research work. A few examples below:
· S. Rosiek, F.J. Batlles. Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building. Renew Energy, 34 (2009), pp. 1423-1431
· O. Marc, F. Sinama, J.-P. Praene, F. Lucas, J. Castaing-Lasvignottes. Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application. Appl Therm Eng, 90 (2015), pp. 980-993
· Z.Y. Xu, R.Z. Wang, H.B. Wang. Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system. Int J Refrig, 59 (2015), pp. 135-143
· Y. Hang, M. Qu, R. Winston, L. Jiang, B. Widyolar, H. Poiry. Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced. Energy Build, 82 (2014), pp. 746-757
· A. Shirazi, R.A. Taylor, G.L. Morrison, S.D. White. Solar-powered absorption chillers: A comprehensive and critical review. Energy Convers Manage, 171 (2018), pp. 59-81
Similarly, the literature review does not take into account the latest works from the discussed issue. Literature review is very modest, it contains only 11 items, of which the latest article is from 2012.
2. The authors describe the cooperation of the system with two heat pumps as a novelty, whereas in the manuscript only one figure and four lines of text were devoted to this issue.
3. Lack of a scientific approach to the described problem. The article presents only dry data from an existing installation.
4. Errors appear in the description of the system scheme (no FZ and T0); incorrect representation of units in table 1
Author Response
Thank you very much for your comments and suggestions. The following major revisions have been made according to your opinions:
1. The English language and style have been improved by MDPI English editing service.
2. Some recent references have been additionally reviewed in the Section 1. Introduction:
• Lines 52-56: S. Rosiek, F.J. Batlles. Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building. Renew Energy, 34 (2009), pp. 1423-1431
• Lines 74-79: O. Marc, F. Sinama, J.-P. Praene, F. Lucas, J. Castaing-Lasvignottes. Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application. Appl Therm Eng, 90 (2015), pp. 980-993
• Lines 79-83: Z.Y. Xu, R.Z. Wang, H.B. Wang. Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system. Int J Refrig, 59 (2015), pp. 135-143
• Lines 69-74: Y. Hang, M. Qu, R. Winston, L. Jiang, B. Widyolar, H. Poiry. Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced. Energy Build, 82 (2014), pp. 746-757
• Lines 101-103: A. Shirazi, R.A. Taylor, G.L. Morrison, S.D. White. Solar-powered absorption chillers: A comprehensive and critical review. Energy Convers Manage, 171 (2018), pp. 59-81
3. A clearer description has been provided of how the system functions in winter and the system performance has been presented in the paper.
Lines 150-154: During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 °C and 60 °C produced by the solar collector field is directly used for building heating. The heat pumps additionally provide hot water when solar energy is insufficient.
Lines 220-235: During the heating period in winter, the valves V1, V2, V7~V10, V13~V18 were open and V3~V6, V11 and V12 were closed. The controlling system was achieved through the following main cycles:
(1) The solar loop
The pump, P1, was switched on when the difference between the outlet temperature of the collector field and the upper layer temperature of the hot water storage tank T3-T4≥4 °C; the pump, P1,was switched off when the temperature difference T3-T4＜2 °C.
(2) The heating loop
The pump, P2, was switched on when the lower layer temperature of the buffer tank T15＜55 °C AND the difference between the upper layer temperature of the hot water storage tank and the lower layer temperature of the buffer tank T4-T15≥2 ; The pump, P2, was switched on when the lower layer temperature of the buffer tank T15≥60 °C OR T4≤T15.
(3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T15＜45 °C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T15≥50 °C OR the fan coils were shut down.
Performance analysis:
Figure 8. The monthly average outdoor temperature and building cooling and heating energy demand in 2018.
Figure 15. The monthly supplied cooling/heating energy, the electricity consumption and the coefficient of performance of the heat pumps COPhp in 2018.
Figure 17. The monthly solar fraction SFn and electricity savings in 2018.
4. The definitions and equations of the solar collector efficiency(lines 262-267), coefficient of performance of the absorption chiller COPchiller (lines 286-295), coefficient of performance of the heat pumps COPhp (lines 324-326) and solar fraction SFn (lines 327-339) have been presented in the paper.
Furthermore, an experimental error has been analyzed, including the errors in all measured variables and the description of the instruments used. Please see the above-mentioned equations and Table 2-The instruments and their calibration range and uncertainty.
5. A comparison between the system performance without and with the heat pumps was presented in the paper.
Lines 350-363: In order to determine the influence of the heat pumps on the system performance, the heat pumps were switched off on 25, 27 and 28 July 2018 and the system performance was analyzed. As shown in Figure 16, the energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. The daily global irradiation onto collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWh on 23 July. However, the collector field yield and cooling energy yield were 183.9 kWh and 133.7 kWh respectively, which were only 60% and 50% of those values on 23 July when incorporated with the heat pumps. The solar faction decreased from 0.62 on 23 July to 0.15 on 28 July. The reason was that it took much more time to produce sufficient chilled water by providing the absorption chiller with hot water by using solar energy than by using both solar energy and the heat pumps.
Figure 16. The irradiation onto collector area, collector field yield, cooling energy yield and solar fraction SFn,c on 23, 25, 27 and 28 July, the heat pumps were switched off on 25, 27 and 28 July,2018.
6. The Section 4: Discussion and conclusion has been revised to get a scientific approach to the described problem, please see lines 384-409.
An absorption solar cooling and heating system assisted by two air-source heat pumps located in Ningbo City, China was studied in this paper. The system was run in 2018 and the operation results were evaluated. Based on this study the main conclusions are as follows:
The solar collector field was comprised of 40 all-glass evacuated tube modules with a total aperture area of 120 m2. The annual average collector efficiency was 44% for building cooling and 42% for building heating.
The single-stage and LiBr–water based absorption chiller had a cooling capacity of 35 kW. The monthly average coefficient of performance COPchiller ranged between 0.68 and 0.76 in 2018. The COPchiller increased with the rising hot water inlet temperature.
Two air-source heat pumps each with a rated cooling capacity of 23.8 kW and heating capacity of 33 kW were used as the auxiliary system for the solar cooling and heating installation. The average coefficient of performance COPhp,c was 2.83 in 2018, with minimums of 2.74 and 2.75 in July and August, respectively. The average coefficient of performance for heating COPhp,h was 3.20.
The energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. In comparison with the case combined with the heat pumps under the similar irradiation condition, the collector field yield and cooling energy yield decreased by more than 40%, and the solar faction decreased from 0.62 to 0.15.
Two kinds of the operation modes were conducted, namely the building cooling and heating modes. The annual mean solar fractions for cooling and heating were 56.6% and 62.5%, respectively. The yearly electricity saving was 10,158.6 kWh when combined with the solar cooling and heating system, which accounted for 41.1% of the total electricity consumption for building cooling and heating and corresponds to 5,445 kg of CO2 emissions prevented from being released into the atmosphere.
1. The work lacks originality, since many research work has been done on solar absorption cooling system. The authors of this work have not even attempted to compare their results with other research work. A few examples below:
· S. Rosiek, F.J. Batlles. Integration of the solar thermal energy in the construction: analysis of the solar-assisted air-conditioning system installed in CIESOL building. Renew Energy, 34 (2009), pp. 1423-1431
· O. Marc, F. Sinama, J.-P. Praene, F. Lucas, J. Castaing-Lasvignottes. Dynamic modeling and experimental validation elements of a 30 kW LiBr/H2O single effect absorption chiller for solar application. Appl Therm Eng, 90 (2015), pp. 980-993
· Z.Y. Xu, R.Z. Wang, H.B. Wang. Experimental evaluation of a variable effect LiBr–water absorption chiller designed for high-efficient solar cooling system. Int J Refrig, 59 (2015), pp. 135-143
· Y. Hang, M. Qu, R. Winston, L. Jiang, B. Widyolar, H. Poiry. Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced. Energy Build, 82 (2014), pp. 746-757
· A. Shirazi, R.A. Taylor, G.L. Morrison, S.D. White. Solar-powered absorption chillers: A comprehensive and critical review. Energy Convers Manage, 171 (2018), pp. 59-81
Similarly, the literature review does not take into account the latest works from the discussed issue. Literature review is very modest, it contains only 11 items, of which the latest article is from 2012.
2. The authors describe the cooperation of the system with two heat pumps as a novelty, whereas in the manuscript only one figure and four lines of text were devoted to this issue.
A comparison between the system performance without and with the heat pumps was presented in the paper.
Lines 350-363: In order to determine the influence of the heat pumps on the system performance, the heat pumps were switched off on 25, 27 and 28 July 2018 and the system performance was analyzed. As shown in Figure 16, the energy yield and solar fraction distinctly decreased without the heat pumps as the auxiliary system. The daily global irradiation onto collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWh on 23 July. However, the collector field yield and cooling energy yield were 183.9 kWh and 133.7 kWh respectively, which were only 60% and 50% of those values on 23 July when incorporated with the heat pumps. The solar faction decreased from 0.62 on 23 July to 0.15 on 28 July. The reason was that it took much more time to produce sufficient chilled water by providing the absorption chiller with hot water by using solar energy than by using both solar energy and the heat pumps.
Figure 16. The irradiation onto collector area, collector field yield, cooling energy yield and solar fraction SFn,c on 23, 25, 27 and 28 July, the heat pumps were switched off on 25, 27 and 28 July,2018.
3. Lack of a scientific approach to the described problem. The article presents only dry data from an existing installation.
The above-mentioned revisions have been made to obtain a scientific approach to the described problem.
4. Errors appear in the description of the system scheme (no FZ and T0); incorrect representation of units in table 1
T0-ambient temperature, FZ-solar radiation, see Figure 6. Schematic of the controlling and monitoring system.
The units in Table 1 have been corrected.
Author Response File: Author Response.doc
Round 2
Reviewer 1 Report
The authors have provided modifications to the paper in their recent revision, and the earlier comments from this reviewer have mostly been addressed. However, some of the new analysis requires clarification as follows:
· Equation 6 describing the solar fraction (SF) in cooling mode: The term “solar fraction” is often applied to solar heating systems, and the definition is problematic for the solar cooling system here because it is given as a ratio of the solar collector heat output to the delivered cooling output and does not account for the intermediate energy conversion process provided by the absorption chiller. Is this definition taken from the literature? If so, a citation should be provided.
· Equation 7 describing the solar fraction (SF) in heating mode: Here the numerator term appears to be based on the delivered energy downstream of the hot water storage tank, rather than upstream of the hot water storage tank (as in Eq. 6). This should be explained in the text. Is it because space heating and hot water provision are considered separately? Please also check the definitions of F3 and F4 provided in the caption of Figure 7, as these do not appear to match watch is shown in the diagram.
· In line 344, the cooling COP is stated to be lowest in July and August. The authors should comment on why this the case, as it is stated earlier in the paper that COP increases with hot water inlet temperature.
· In line 360, it is stated that the solar fraction in cooling mode dropped from 0.62 to 0.15 when the auxiliary cooling source (the air source heat pumps) were disabled. The reason for this is not clearly explained. A lower solar fraction suggests that solar energy is now accounting for an even lower percentage (15%) of the delivered cooling. If the heat pumps are disabled, then what is accounting for the other 85% of the energy input? Is there another auxiliary energy source? Or is it due to the problem with the definition of the solar fraction in cooling mode mentioned above (see first bullet point). This requires clarification.
Author Response
Thank you very much for your comments and suggestions. The following revisions were made according to your opinions:
· Equation 6 describing the solar fraction (SF) in cooling mode: The term “solar fraction” is often applied to solar heating systems, and the definition is problematic for the solar cooling system here because it is given as a ratio of the solar collector heat output to the delivered cooling output and does not account for the intermediate energy conversion process provided by the absorption chiller. Is this definition taken from the literature? If so, a citation should be provided.
Sorry there was a mistake in this equation and it has been corrected, please see lines 325-340. The solar fraction SFn is defined as the ratio of the generated cooling or heating by solar energy to the total generated cooling or heating energy by solar and the heat pumps which corresponds to the total cooling or heating energy used by the fan coils. The equation was modified and presented in lines 328-329.
where Qt, Qc and Qh are the total cooling or heating energy used, cooling energy generated by the chiller driven by solar and heating energy generated by solar, respectively; F2, F3 and F4 are the hot water inlet flow rate of the absorption chiller, chilled water inlet flow rate of the absorption chiller and inlet flow rate of the fan coils, respectively; cp,w is the specific heat of water; T4 and T5 are the upper layer temperature and lower layer temperature of the hot water storage tank, respectively; T10 and T11 are chilled water outlet and inlet temperature of the absorption chiller, respectively.
· Equation 7 describing the solar fraction (SF) in heating mode: Here the numerator term appears to be based on the delivered energy downstream of the hot water storage tank, rather than upstream of the hot water storage tank (as in Eq. 6). This should be explained in the text. Is it because space heating and hot water provision are considered separately? Please also check the definitions of F3 and F4 provided in the caption of Figure 7, as these do not appear to match watch is shown in the diagram.
There is no hot water demand in the building (this sentence has been added in line 131). Therefore, the hot water provision was NOT considered in this paper.
The delivered energy downstream of the hot water storage tank was considered, rather than upstream of the hot water storage tank, because the downstream of the hot water storage tank was directly pumped to the buffer storage for building heating. The flow rate of the downstream did NOT correspond to that of the upstream because the hot water was pumped to the buffer storage only when the lower layer temperature of the buffer tank T15＜55 oC AND the difference between the upper layer temperature of the hot water storage tank and the lower layer temperature of the buffer tank T4-T15≥2 oC (lines 227-229).
F3 is the chilled water inlet flow rate of the absorption chiller; F4 is the inlet flow rate of the fan coils. In the equations (5) and (6), the cooling energy generated by the chiller driven by solar and the total generated cooling or heating energy by solar and the heat pumps which corresponds to the total cooling or heating energy used by the fan coils .
· In line 344, the cooling COP is stated to be lowest in July and August. The authors should comment on why this the case, as it is stated earlier in the paper that COP increases with hot water inlet temperature.
In line 344, the cooling COP is the COPhp of the two air-source heat pumps, which stated to be lowest in July and August.
In lines 306-307, it was found that the COPchiller increased with the rising hot water inlet temperature, which is the COPchiller of the absorption chiller.
· In line 360, it is stated that the solar fraction in cooling mode dropped from 0.62 to 0.15 when the auxiliary cooling source (the air source heat pumps) were disabled. The reason for this is not clearly explained. A lower solar fraction suggests that solar energy is now accounting for an even lower percentage (15%) of the delivered cooling. If the heat pumps are disabled, then what is accounting for the other 85% of the energy input? Is there another auxiliary energy source? Or is it due to the problem with the definition of the solar fraction in cooling mode mentioned above (see first bullet point). This requires clarification.
You have right. There was no another auxiliary energy source except for the two heat pumps. We conducted the test only to determine the influence of the heat pumps on the system performance. The solar fraction SF should be 100% without any auxiliary energy source according to the definition in cooling mode mentioned above. This result can NOT be used for the comparison and I have deleted the SF in Fig.16 and in the test in lines 359-360.
Author Response File: Author Response.doc
Reviewer 2 Report
The Authors modified the revised version that could be accepted in present form now
Author Response
Thank you very much.
Reviewer 3 Report
Authors addressed all the criticisms raised. Paper can be published in the present form.
Author Response
Thank you very much.
Reviewer 4 Report
The authors have made suggested corrections. The article in its current form can be published.
Author Response
Thank you very much.
