Elsevier

Renewable Energy

Volume 148, April 2020, Pages 558-572
Renewable Energy

Performance assessment of a hybrid photovoltaic-thermal and heat pump system for solar heating and electricity

https://doi.org/10.1016/j.renene.2019.10.061Get rights and content

Highlights

  • A Hybrid Solar Photovoltaic/Thermal Heat Pump system is modelled.

  • Variations in solar irradiation, PVT water flow rate and tank sizes were analysed.

  • Laminar and turbulent flow regimes through the PVT water pipe were analysed.

  • Increasing the flow rate from 3 L/min to 17 L/min reduces the PVT temperature by 3 °C.

  • Increasing tank volume from 1 L to 100 L increases the PVT total efficiency by 6.5%.

Abstract

This work investigates a solar combined heat and power systems based on hybrid photovoltaic-thermal heat pump systems for the simultaneous provision of space heating and electricity to residential homes. The analysed system connects a photovoltaic-thermal (PVT) panel, through a PVT water tank, to a heat pump. The study is based on quasi-steady state heat transfer and thermodynamic analysis that takes incremental time steps to solve for the fluids temperature changes from the heat pump and the solar PVT panels. The effects of solar irradiance, size of the water tank and the water flow rate in the PVT pipes (laminar and turbulent) on the performance of the system are analysed. Particular focus is made towards the efficiency (electrical and thermal) of the PVT and the COP of the heat pump. Results show that the minimum COP of the heat pump is 4.2, showing the high performance of the proposed hybrid system. Increasing the water flowrate through the PVT panel from 3 L/min (laminar) to 17 L/min (turbulent) increases the PVT’s total efficiency (electrical + thermal) from 61% to 64.5%. Increasing the size of the PVT water tank from 1 L to 100 L, increases the total efficiency of the PVT panel by 6.5%.

Introduction

A change towards the de-carbonisation and diversification of energy sources is taking place globally [1]. The overall movement is towards renewable and sustainable energy, including solar energy [2]. In this regard, solar photovoltaics (PV) are extensively used to generate electricity [3]. However, PV panels are typically 20% efficient [4]. The rest of the absorbed sunlight is converted into heat [4]. The generated heat increases the temperature of the panel, resulting in a decrease in electrical efficiency [5]. This generated heat must be extracted from PV panels to prevent excessive heating of the PV cells. Panels can be actively cooled by passing a fluid through the rear of the panel to extract both heat and electrical power [6]. This combined solar heat and electrical power system is known as a photovoltaic-thermal (PVT) system [3]. The fluid that passes through the PVT panel absorbs the excess heat, reducing the PV temperature [7]. The heated fluid is used for heat related energy consumption (e.g. Refs. [5,8]). Herrando et al. [5,9] using thermodynamic modelling showed that a PVT system could cover 51% of the electrical demand and 36% of the hot water demand for a 3-bedroom house in London, UK. However, the greatest domestic energy consumption is heating [1]. In Europe, buildings consume 60% of their total energy for space heating [1]. The challenge is that the heat energy recovered from the PV panel does not directly produce high enough temperatures to cover the heating demand of a household. One solution to this challenge is to integrate the PV panel with a heat pump [10]. An area of research with this technology is in direct expansion PVT heat pump (DEPVT/HP) systems. This technology involves the direct heating of the heat pump’s working fluid by PVT panels, which has been extensively researched in previous numerical (e.g. Refs. [10,11]) and experimental (e.g. Refs. [[12], [13], [14], [15]]) studies. A cooled PV based on a DEPVT/HP system can have up to 2% higher electrical efficiency than the uncooled PV module [12] and can achieve a relatively a high combined coefficient of performance (COP1) of 5.6 [15].

From a practical point of view, installing a DEPVT/HP system on a domestic site can become a health and safety hazard [16]. In homes, solar PVT panels are usually installed on the roof [17]. For a DEPVT/HP system, the heat pump refrigerant will have to circulate outside the heat pump unit towards the PVT through connecting pipes, and then return to the heat pump unit [18]. The extra piping required would make system installation difficult as the piping needs to meet the sealing standards for refrigerants [18]. This refrigerant piping would experience varying temperatures and pressures as the heat source varies throughout the operation of the heat pump [18]. This may result in possible refrigerant leaks [18], which can present health risks for the occupants [16,19] and can contribute to climate change [20]. These issues make the DEPVT/HP impractical for deployment in domestic applications.

A solution to the problems associated with the DEPVT/HP systems is the utilisation of indirect expansion PVT heat pump (IEPVT/HP) systems [18]. An IEPVT/HP system uses a fluid (e.g. water) to absorb the solar thermal energy from a PVT panel and cycle it to a heat exchanger to transfer heat to a heat pump cycle or store in a water tank [18,21]. The water tank acts as a heat source for the heat pump [18,21]. Besgani et al. [22] conducted an experimental study on a dual-source solar-assisted IEPVT/HP system in Milan, Italy, on a detached prefabricated building. Seven PVT panels and one PV panel were used to compare the two different technologies. The PVTs were cooled using water and transferred to the heat pump via a water-based evaporator. The heat pump also used an air-based evaporator to use air as another heat source. They [22] found that the “water-source” operation of the heat pump outperformed the “air-source” operation by 34%, and that the water-source heat pump did not require any defrost cycling. It was also observed that the electricity production of the PV and PVT panels were similar [22]. They [22] concluded that their IEPVT/HP system has an average COP of 3 [22]. In another experimental study in Lyngby, Denmark, Dannemand et al. [23] analysed the performance of a solar IEPVT/HP system for nine months. They [23] demonstrated that their system can operate and absorb solar energy at solar radiation intensities greater than 50 W/m2 and act as an air source heat absorber at solar radiation intensities less than 50 W/m2 [23]. Though the system was proved to work, the researchers concluded that optimisation of the system is important [23].

In comparison to the DEPVT/HP, research in the IEPVT/HP is sparse. In literature, the majority of research studied the medium and long term (e.g. months) operation of different configurations of IEPVT/HP systems [[22], [23], [24], [25]]. Additionally, the influence of the system pertinent parameters (variation in the solar irradiation, PVT water flow rate and storage tank) on the response of the system for long-term operation has not been well studied. Thus, the body of knowledge in this area lacks documentation on the short-term changes that occur with intermittent energy sources such as solar energy. Furthermore, the effect of solar energy intermittency on the short-term (e.g. hours) operation of the IEPVT/HP system has not been analysed. Hence, the main objective of the present work is to observe the effects of variation in the solar irradiation, PVT water flow rate and water storage tank volume on the short-term operation of an IEPVT/HP system. Such an analysis enables us to understand the system’s response to the transient variations of different parameters affecting the performance of the IEPVT/HP system. Short-term analysis allows us to understand, (i) the influence of the intermittency on the system’s electrical and thermal performance [26], (ii) analyse the capability of the system’s flexible elements (e.g. water flow rate and storage tank size) to suppress the solar energy intermittency, and (iii) optimise the design of the system’s parameters in order to minimise the impact of the intermittency on the long-term operation of the system. This will eventually contribute to a smarter design of control systems for such technologies for domestic applications. Therefore, this study analyses the thermal and electrical performance of an IEPVT/HP system under short-term operation, by analysing the variation of key parameters, which control the performance of a hybrid system, including solar irradiance, water flow rate in the PVT and storage tank size.

Section snippets

System configuration

The system configuration in this work consists of (from left to right) a PVT water loop, a PVT water tank loop, the water-to-water heat pump loop and a heat rejection loop, as shown in Fig. 1. The water-to-water heat pump loop consists of an evaporator, compressor, condenser and expansion valve. The heat rejection loop consists of a water tank to supply the condenser, a heat pump condenser, and a forced convection radiator that rejects heat to the user.

The operation of the system in Fig. 1 is

Mathematical modelling

The mathematical model of the system is based on equations representing the thermodynamic and heat transfer processes occurring in the system. The model is a quasi-steady state model that takes incremental time steps to solve for the fluid temperature changes within the system. MATLAB code was developed to solve the system of governing equations. The Runge-Kutta 4th order method was employed to solve the PVT energy balance equations. The heat pump equations were iterated to a solution within a

Validation

In this study, the two main parts of the system (i.e. the PVT and the heat pump) are validated against numerical and experimental data recorded in the literature.

Results and discussion

The system modelled in Fig. 1 is used to simulate the heating of a 5m × 3m × 3m space using a radiator of dimension 2m × 0.15m × 0.15m. The radiator uses forced convection with an air velocity of 0.5 m/s. The room starts at an ambient temperature of 14 °C (i.e. the average summer temperature in Belfast, UK [38]) and the system operates for 3600 s. The initial water temperatures in the condenser water tank and PVT water tank are considered to be the same as ambient air temperature. The condenser

Conclusions and further discussion

In this work, the thermal and electrical performance characteristics of a hybrid photovoltaic-thermal heat pump system were studied using thermodynamic and heat transfer analysis. The study focused on the quasi-steady state modelling of an Indirect Expansion PVT Heat Pump (IEPVT/HP) configuration. The simulations demonstrated the system’s transient performance characteristics for various solar irradiances, laminar and turbulent flow regimes in the PVT water pipe, and size of the water storage

Acknowledgement

This proejct is partailly funded by the BL Refrigeration and Air Conditioning Ltd under the grant No. R5036MEE.

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