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A systematic review of recent air source heat pump (ASHP) systems assisted by solar thermal, photovoltaic and photovoltaic/thermal sources
Xinru Wang, Liang Xia, Chris Bales, Xingxing Zhang, Benedetta Copertaro, Song Pan, Jinshun Wu
TL;DR
Solar-assisted ASHP systems vary in configurations, parameters, and performance criteria, complicating comparison and implementation. This paper systematically reviews three solar-assisted ASHP categories and finds differing performance, investment, and payback characteristics.
Problem
Solar-assisted ASHP systems vary with boundary conditions and characteristic parameters, while comparative understanding remains unclear.
Method
The paper conducts a systematic review of three solar-assisted ASHP systems and describes their configurations and implementation considerations.
Results
PV-ASHP has the highest mean COP at 3.75, compared with 3.03 for PV/T-ASHP and 2.9 for ST-ASHP; ST-ASHP requires smaller investment, while PV/T-ASHP requires larger capital cost and longer payback time.
Takeaways & Limitations
The comparison provides information for choosing and implementing an appropriate solar-assisted ASHP system in practice.
Takeaways & Limitations
A key research gap is the challenge of bridging solely simulation or laboratory testing and real application.
Abstract
from arXiv · showhide
The air source heat pump (ASHP) systems assisted by solar energy have drawn great attentions, owing to their great feasibility in buildings for space heating/cooling and hot water purposes. However, there are a variety of configurations, parameters and performance criteria of solar assisted ASHP systems, leading to a major inconsistency that increase the degree of complexity to compare and implement different systems. A comparative literature review is lacking, with the aim to evaluate the performance of various ASHP systems from three main solar sources, such as solar thermal (ST), photovoltaic (PV) and hybrid photovoltaic/thermal (PV/T). This paper thus conducts a systematic review of the prevailing solar assisted ASHP systems, including their boundary conditions, system configurations, performance indicators, research methodologies and system performance. The comparison result indicates that PV-ASHP system has the best techno-economic performance, which performs best in average with coefficient of performance (COP) of around 3.75, but with moderate cost and payback time. While ST-ASHP and PV/T-ASHP systems have lower performance with mean COP of 2.90 and 3.03, respectively. Moreover, PV/T-ASHP system has the highest cost and longest payback time, while ST-ASHP has the lowest ones. Future research are discussed from aspects of methodologies, system optimization and standard evaluation.
Content
The paper uses abbreviations for heat-pump configurations, solar technologies, performance indicators, and assessment methods.
- ASHP denotes air source heat pump, while SAHP denotes solar assisted heat pump.
- COP, EER, and SPF denote coefficient of performance, energy efficiency ratio, and seasonal performance factor.
- LCA, EROI, EIF, TCC, and TTCC denote life cycle assessment, energy returned in invested, emission intensity factor, tolerable capital cost, and total tolerable capital cost.
- ST-ASHP, PV-ASHP, and PV/T-ASHP identify solar thermal, photovoltaic, and photovoltaic/solar thermal assisted air source heat pumps.
1 Introduction
The introduction motivates solar-assisted ASHPs by cold-weather performance challenges and establishes a systematic comparison of ST, PV, and PV/T configurations. It also defines the review scope and its emphasis on practical implementation.
- ASHP performance drops significantly at low temperatures, with frosting possible under severe conditions.
- Solar-assisted ASHPs combine air-source heat pumps with solar thermal collectors, photovoltaic arrays, or photovoltaic/thermal systems.
- The review addresses a comparison problem arising from diverse configurations, boundary conditions, parameters, and performance outcomes.
- The study systematically reviews recent research on three solar-assisted ASHP types and compares their methodologies, performance, advantages, disadvantages, and applications.
- The review considers only air source heat pumps combined with solar thermal, photovoltaic, or photovoltaic/thermal sources at the system level.
2.1 System boundaries
System boundaries determine which components and energy flows are included when solar-assisted ASHP performance is assessed. The review maps Task 53 boundary categories onto ST-ASHP, PV-ASHP, and PV/T-ASHP systems.
- System boundaries are important for understanding solar-assisted ASHP systems and evaluating their performance.
- Researchers often fail to define system boundaries clearly, making different systems difficult to distinguish.
- Task 53 distinguishes boundaries for the heat pump alone, solar-assisted systems, systems including circulation pumps, PV-array systems, and battery-equipped building systems.
- The PV-array boundary excludes battery storage, whereas the building boundary includes battery storage for heating, domestic hot water, and electricity.
- Within the review, SHP+, SHPPV, and SHPBLDG correspond to ST-ASHP, PV-ASHP, and PV/T-ASHP, respectively.
2.2 System configuration
The review classifies solar-assisted ASHPs by evaporator-related solar source and by system interaction, with ST-ASHP, PV-ASHP, and PV/T-ASHP supporting different energy services and configurations.
- The review categorizes solar-assisted ASHP systems according to the evaporator component as ST-ASHP, PV-ASHP, and PV/T-ASHP.
- ST-ASHP system: ST-ASHP systems include direct-expansion and indirect-expansion designs, with indirect systems classified as parallel, serial, regenerative, or complex.
- ST-ASHP system: Parallel ST-ASHP systems allow the solar collector and air source heat pump to independently supply heating, domestic hot water, or cooling.
- PV-ASHP system: PV-ASHP systems can provide heating, cooling, and electricity, and are classified into direct- and indirect-expansion types and by array type.
- PV/T-ASHP system: PV/T systems produce electricity and heat, can cool photovoltaic cells, and may supply heating and domestic hot water when combined with ASHPs.
- PV/T-ASHP system: PV/T systems include air-cooled, water-cooled, combined air-and-water, PCM, and heat-pipe configurations, with several building-integration options.
2.3 System performance indicators
The review assesses solar-assisted ASHP systems using energetic, economic, and environmental indicators within defined boundaries and operating conditions. It emphasizes inconsistent measurement practices and limited building-level reporting, which hinder comparisons across systems.
- Energetic performance indicators: COP, EER, and SPF are the main energetic indicators, with EER used for cooling and COP or SPF for heating and hot water.SPF includes overall useful supplied energy and electricity used during operation, including auxiliary components such as storage systems.
- Energetic performance indicators: Building-level EER, COP, and SPF data remain very limited, requiring greater attention in future research.The review identifies a shortage of energetic performance indicators at building level compared with heat-pump-unit measurements.
- Energetic performance indicators: System boundaries determine which electrical loads enter COP and SPF calculations, including heat-pump, solar-circuit, storage, control, and circulation-pump consumption.The reviewed definitions distinguish heat-pump electricity from auxiliary and circulation electricity across system configurations.
- Economic performance indicators: Economic assessment commonly considers capital investment, payback time, EPBT, EROI, and life-cycle cost, but studies differ in which costs and indicators they include.LCC can combine system, installation, maintenance, and energy-use costs, while TCC estimates recoverable upgrade cost from annual savings and allowed payback.
- Economic performance indicators: Initial investment estimates are uncertain because installed costs vary with equipment scope, location, market conditions, site requirements, and labour rates.The review therefore describes an alternative tolerable-capital-cost approach for economic feasibility analysis.
- Environmental performance indicators: Environmental assessment is less standardized: few studies report environmental performance in detail, and differing calculation methods and system emphases complicate comparison.Reported measures include CO2e and other emissions, with electricity-related factors varying between peak and base periods.
3 Simulation researches and the related results
Simulation studies of solar-assisted ASHP systems vary in models, operating conditions, input parameters, and performance indicators. Research mainly optimizes system configurations and evaluates COP, SPF, energy use, cost, solar fraction, and environmental performance across ST-, PV-, and PV/T-assisted systems.
- Methods: TRNSYS is the dominant simulation platform, supplemented by CFD, self-developed mathematical models, Matlab, and other component-based approaches.Models represent heat pumps, collectors, heat exchangers, storage tanks, weather data, and multi-zone buildings.
- Methods: Solar irradiation, ambient air temperature, location, operating mode, and component design parameters are the principal boundary and input conditions.Evaluated modes include heating, cooling, domestic hot water, and PCM charging or discharging.
- Evaluation: COP, SPF, payback time, greenhouse-gas emissions, electricity use, solar fraction, and cost are commonly used to assess system performance.Simulation studies also report output variables such as domestic-hot-water production, discharge energy, collector area, heat-exchanger UA-value, and heat-pump size.
- Results: 14.2% electricity savings and a 71.1% solar fraction were achieved by an optimized solar-assisted system.The reported results associate optimization with reduced electricity use and increased solar contribution under the studied boundary conditions.
- Results: PV/T-assisted systems increased COP from 2.74 to 3.45 and reduced winter heat-pump electricity consumption by 20%.The integrated BIPV/T+ASHP system also increased overall COP and reduced greenhouse-gas emissions and energy consumption.
4 Experiment methodologies and the related results
The review summarizes experimental methods, operating conditions, measured parameters, and performance results for solar-assisted ASHP systems. Results show that performance depends on operating mode and environmental conditions, with reported gains in COP, energy use, and exergy performance.
- SIASHP results: 2.54 average IPLV and 2.53 average SPLV for SIASHP were 14.9% and 15.5% higher than ASHP, respectively.The reported SIASHP values were based on Turkey weather data.
- Solar and CO2 heating: 13.5 and 2.18 were the system COPs for solar heating and CO2 heat-pump heating modes, respectively.The solar-assisted system also saved 53.6% of electricity consumption relative to CO2 HP.
- Methods and evaluation: COP, SPF, power consumption, and payback time are the main evaluation parameters used in experimental studies.Experiments commonly measure component temperatures and pressures, input power, and flow rate under different operation modes and outdoor conditions.
- Cold-condition performance: At -15 ºC ambient temperature, solar-assisted ASHP heat capacity and COP increased 62% and 59% compared with ASHP.Other experiments found frosting depended on ambient temperature, humidity, and solar irradiation.
- PV-ASHP results: 3.75 was the reported average heat-pump COP after increasing from 3.6, with a 5% primary-energy reduction relative to the reference case.The integrated PV system’s performance depended on environmental conditions including solar radiation, outlet temperature, and load factor.
- PV/T-ASHP results: 3.03 was the average heat-pump COP for the PV/T-air composite system, while comprehensive COPsys averaged 2.99.The reported heat-pump COP ranged from 5.61 to 1.69, and COPsys ranged from 6.07 to 1.33.
5 Comparison of different solar assisted ASHP systems
The three solar-assisted ASHP configurations differ in energy outputs, environmental sensitivity, control complexity, installation requirements, and payback time. ST-ASHP has the shortest payback, PV-ASHP the strongest performance, and PV/T-ASHP the broadest energy production but greatest complexity and longest payback.
- Common limitations: All solar-assisted ASHP systems require higher investment, installation, and maintenance costs and more complex operation than conventional ASHP.These systems therefore involve a trade-off between broader renewable-energy use and greater system complexity.
- ST-ASHP: ST-ASHP has the shortest payback time among the three systems and can avoid frosting in winter, even below ambient freezing conditions.Its installation requirements are less strict than those of PV-ASHP and PV/T-ASHP.
- ST-ASHP: ST-ASHP mainly produces energy for heating or hot water and is more affected by ambient temperature than PV-ASHP and PV/T-ASHP.The other two systems mainly rely on solar radiation, whereas ST-ASHP is significantly related to ambient temperature.
- PV-ASHP: PV-ASHP can produce electricity for direct building use in addition to heat and hot water, but requires more complex control and electricity storage.Control must coordinate PV generation, grid import, batteries, or thermal energy storage.
- PV/T-ASHP: PV/T-ASHP produces heat, hot water, and electricity with the highest solar utilization, but has the most complex control and longest payback time.Its maintenance and operation are more difficult than for the other solar-assisted ASHP systems.
6 Limitations and future directions
Solar-assisted ASHP research remains constrained by inconsistent methods, complex controls, limited validation, and nonstandardized evaluation. Future work should improve system optimization, practical deployment, common testing, field validation, and comparative standards.
- Ambient temperature and solar irradiation affect system operation, while complex control and high installation and maintenance demands constrain deployment.These limitations are identified across solar-assisted ASHP systems.
- Methodologies vary in experiment modes, boundary conditions, and model choices, limiting consistent performance comparison and marketization.The review highlights inconsistency between studies and a gap between simulation or laboratory testing and real application.
- Common simulation tools, standard testing conditions, pilot-project field data, and validation of simulation or experiment results are needed.Common standards and official certifications could support user choice and comparison among solar-assisted ASHP systems.
- PV/T-ASHP experiments are covered by few papers, with most studies relying on simulation; practical technical or installation problems remain before wider application.The review identifies PV/T-ASHP as not yet largely applied.
- Future optimization should address system structure, component sizing, operation modes, and electricity storage capacity or volume.The review notes that time-adjusted operating modes may be inaccurate and calls for optimization across different components.
- COP and SPF are widely used, but differing processes and evaluation parameters show a lack of standardized performance indicators.Environmental and economic parameters are also considered in few papers.
- Combining solar-assisted ASHP systems with other systems may avoid single-system disadvantages and provide more building energy-system solutions.The review gives heat recovery and solar ASHP combination as an example of prospective development.
7 Conclusions
The review compares three solar-assisted ASHP configurations using system boundaries, configurations, methods, performance indicators, and reported performance. PV-ASHP has the highest mean COP, while ST-ASHP requires the smallest investment and PV/T-ASHP faces the greatest cost and payback burden.
- Methodologies: System boundaries should be defined from the outset according to IEA SHC Task 44/HPP Annex 38 for standardized performance evaluation.The review notes that experiments are commonly conducted in laboratories and simulations are widely used.
- Performance evaluation: COP is the most commonly adopted indicator for evaluating solar-assisted ASHP performance, while weather data strongly affects results.Relevant weather variables include temperature and solar radiation.
- Comparative performance: 2.9 is the ST-ASHP mean COP, with relatively low performance but smaller investment and a 3.5-10-year payback time.ST-ASHP is also the most studied of the three configurations.
- Comparative performance: 3.75 and 3.03 are the reported mean COP values for PV-ASHP and PV/T-ASHP, respectively, with higher capital cost and longer payback associated with the latter systems.The comparison identifies PV-ASHP and PV/T-ASHP as having somewhat higher COP than ST-ASHP, while requiring greater capital investment or longer payback.
- Future research: Future work should develop common simulation tools, standard testing conditions, field data, optimized system structures and controls, and common indicators or certifications.The review also recommends pilot projects, validation of simulation and experiment results, and combinations of system concepts.
- Future research: Common standards and official certifications could improve user choice and comparison among solar-assisted ASHP systems.The review presents these measures as part of broader efforts to clarify current R&D status and promote building-level application.