Elsevier

Solar Energy

Volume 197, February 2020, Pages 279-291
Solar Energy

Performance analysis of solar assisted heat pump coupled with build-in PCM heat storage based on PV/T panel

https://doi.org/10.1016/j.solener.2020.01.002Get rights and content

Highlights

  • A solar PV/T heat pump is proposed for residential heating in high latitude area.

  • Design of build-in PCM heat storage unit is proposed and simulated.

  • The overall efficiency of this solar PV/T heat pump system can reach above 75%.

Abstract

PV/T (photovoltaic/thermal) technology is a combination of PV module (photovoltaic utilization) and collector (photothermal utilization), which can improve the comprehensive utilization efficiency of solar energy and has a broad application prospect. In this paper, PV/T module is coupled with heat pump evaporator to form a direct-expansion solar PV/T heat pump which is suitable for heat application in high latitude area. To achieve stable residential heating, a solar PV/T heat pump system coupled with build-in PCM (phase change material) heat storage is therefore proposed and simulated. Meanwhile, the mathematical model of solar PV/T heat pump coupled with build-in PCM heat storage system is established and verified. The simulation results show that the temperature of underfloor heating which using build-in PCM heat storage can reach 22–31 °C after 39 h when the circulating water is 40 °C. Moreover, the heating COP (Coefficient of Performance) increases with the increase of solar radiation, ambient temperature and area of PV/T collector, and decrease of wind speed, respectively. A 20 m2 PV/T panel module can output 21.4% of the electricity to power grid when the solar radiation intensity is 600 W/m2 and meet the heat demand of a 100 m2 room while maintain the operation of the system. Meanwhile, the heating COP can reach 5.79 which is 70% higher than the conventional air conditioning system and the electrical, thermal, overall efficiencies are 17.77%, 55.76% and 75.49%, respectively.

Introduction

The total amount of energy consumption in the world is constantly climbing (Caetano et al., 2017). The consumption of fossil energy has brought about energy crisis and environmental crisis (Pietrosemoli and Rodríguez-Monroy, 2019). Without action, CO2 emissions from burning fossil fuels will be doubled by 2050 (Paolo Frankl, 2010). Therefore, the development and utilization of renewable energy has become one of the effective solutions (Keček et al., 2019). Solar energy has become the first choice due to its characteristics of ubiquity, abundance and sustainability (Kuik et al., 2019, Tsai, 2015), which is mainly used in two ways: photothermal and photovoltaic. 11% of global electricity will be provided by PV by 2050 (Paolo Frankl, 2010). However, the electrical efficiency of PV cells decreases with the increase of the temperature of PV cells (Huide et al., 2017). A cooling system can be added to reduce the temperature of PV cells while the remaining heat of PV panel are absorbed by working fluid which can be employed as a useful thermal energy for heat applications in buildings.

The PV/T technology coupled PV modules with thermal collectors was first proposed by Wolf (1976) to reduce PV cells temperature and improve electrical efficiency. The PV/T system can recover waste heat from the PV panel to improve comprehensive energy utilization efficiency. PV/T design optimizations are carried out to improve the system efficiency in recent years. Nahar et al. (2017) designed a novel pancake-shaped flow channel for PV/T system, and integrated the flow channel with the PV baseboard. They found that the temperature of the PV panel is reduced by 42 °C, and the electrical efficiency is increased by 2%. Othman et al. (2016) proposed a parallel, double pass flat plate collector which was adopted in a two fluids PV/T system. Their results showed that the electrical efficiency and thermal efficiency are 17% and 76%, respectively.

The combination of PCM and PV/T panel is an effective way to stabilize the operating temperature of PV cells and improves the overall efficiency. Hosseinzadeh et al. (2018) investigated the effect of simultaneous use of nanofluid as coolant as well as an organic paraffin as the phase change material on the electrical and thermal efficiencies. They demonstrated that the use of PCM in nanofluid based PVT/PCM system enhances the thermal output power of conventional PV/T system by 29.6%. Kazemian et al. (2019) developed and simulated a comprehensive three-dimensional model of PV/T system integrated with PCM. Their simulation results presented that the PV/T-PCM system have lower surface temperature compared to PV/T system, and as the thermal conductivity of PCM enhances, both electrical and thermal efficiencies increase. Fayaz et al. (2019) investigated the PCM based PV/T system, and the experimental validation was carried out to verify the numerical model. They found that the electrical efficiency is achieved as 13.98% and 13.87% numerically and experimentally respectively, and the electrical performance is improved as 6.2% and 4.8% for PV/T-PCM system based on the numerical and experimental results respectively.

Different working fluids like water, air, nanofluid and refrigerant are also used to cool the PV module. Huang and Lee (2004) conducted long-term tests on the direct-expansion solar heat pump which adopted refrigerant as working fluid to verify the stability of the work. The total running time of their prototype is over 20,000 h, and the measured energy consumption is 0.019 kWh/l of hot water at 57 °C which is much less than traditional solar water heater. Stojanović and Akander (2010) used direct-expansion heat pump for independent buildings heating and domestic hot water supply. In their system, the collector area is 42.5 m2 and heat pump power is 8.4 kW, they measured that the actual indoor temperature is no less than 20 °C during the testing period. Del Amo et al. (2019) verified the feasibility of solar PV/T heat pump through experiments. They obtained that the highest COP of the system can reach 4.62. Meanwhile, the PV module provides 67.6% of the power demand, and the payback period is 6 years.

In addition to optimize the PV/T panel, the adoption of PCM as heat storage is also a good way to stabilize the system. Kuznik et al. (2008) adopted PCM wallboard heat storage and conducted comparative experiments. In their study, the system can effectively reduce heat loss, keep the room warm and improve indoor thermal comfort. Fiorentini et al. (2015) combined PCM storage with PV/T system, and the roof was used as PV/T layout location. The PCM storage adopted in their system can keep indoor comfort within a certain and potentially variable thermal comfort range. Diallo et al. (2019) proposed the PVT-LHP (PVT Loop Heat Pipe) technology employing PCM triple heat exchanger, the total energy efficiency of the presented system is improved by 28%, and the heating COP is 2.2 times than that of a traditional PV/T system.

Owing to the instability of solar energy, traditional solar PV/T system cannot continuously and stably supply heat or power generation when solar irradiation is weak such as rainy day or winter. Consequently, the market of PV/T technology compared with PV or PT system is still very low. PV/T can adapt to the characteristics of low intensity, instability and intermittency of solar energy better if it can be combined with accumulator and heat storage. However, additional space is required to install heat storage tank, which is not suitable for use in urban areas where land resources are scarce. Therefore, in this paper, a coupling design of solar PV/T heat pump and build-in PCM heat storage is proposed and the parallel air source heat exchanger is also adopted to enhance the stability of the system. The build-in PCM heat storage used for underfloor heating is a combination of PCM and building materials, which can save more space compared to conventional PCM storage tank system. Firstly, the composition and operation modes of the system are introduced. According to the system principle, the mathematical model is established and verified, and the build-in PCM heat storage sub-system which using for residential heating is also proposed and simulated. Then the influences of different parameters on system performance are analyzed. Finally, the feasibility analysis of the system is conducted. The objective of this paper is to provide a promising method to realize stable, high efficiency, environmental friendly residential heating in high latitude area with no energy consumption from power grid.

Section snippets

System description

Fig. 1 shows the schematic diagram of the system based on solar PV/T heat pump, which is consisted of four main parts: solar PV/T heat pump module, parallel air source heat pump module, heat storage module and electrical module. The blue lines represent low temperature working fluid, and in the opposite, red lines represent high temperature. The yellow lines represent the electricity flow direction. The arrows show the working fluid direction. The system can be divided into two operating modes,

Mathematical model

The thermodynamic state points for each process are shown in Fig. 2. The solar PV/T heat pump cycle could be simplified to four components: PV/T collector/evaporator, compressor, PCM heat exchanger and throttle valve. Different temperature (T) and enthalpy (h) at each state point are shown in Fig. 2. Qth (W) is the heat transfer rate between refrigerant and the PV/T panel, Qe (W) is the electrical power provided by PV panel, and Qcond (W) is the heat transfer rate in condenser.

The design

Validation of the model

To ensure the reliability of the mathematic model, the simulation results should be compared with the experimental results. The experimental parameters used in the simulation are listed in Table 3.

The comparison results of heating COP are presented in Fig. 11(a), the operating conditions are refer from Zhou et al. (2019). Under the same system components (PV/T collector/evaporator, compressor, condenser, expansion valve, water tank), the simulation results are in good agreement with the

Parameter analysis

In this section, the influences of different parameters (solar radiation intensity, ambient temperature, wind speed, area of PV/T collector) on this system are investigated, and the performance indices of the system under typical working conditions are also given. It should be noted that when one parameter is varied, others keep constant. Pressure ratio of the compressor refers the ratio of pressure of discharged refrigerant vapor and charged refrigerant vapor.

Conclusion

A building-coupled cogeneration system using solar PV/T heat pump and build-in PCM heat storage is proposed in this paper. The mathematical model of the system is established and verified to analyze the system performance under different conditions. The main conclusions can be drawn as follows:

  • (1).

    The temperature of underfloor heating which using build-in PCM heat storage can reach 22 °C to 31 °C after 39 h when the circulating water is 40 °C which is stable and suitable for residential heating.

  • (2).

    The

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research work is funded by the International Research Cooperation Program of Shanghai (Grant No. 18160710500).

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