Hybrid photovoltaic-thermal (PVT) collectors have been proposed for the combined generation of electricity and heat from the same area. In order to predict accurately the electrical and thermal energy generation from hybrid PVT systems, it is necessary that both the steady-state and dynamic performance of the collectors is considered. This work focuses on the performance characterisation of non-concentrating PVT collectors under outdoor conditions. A novel aspect concerns the application of existing methods, adapted from relevant international standards for flat plate and evacuated tube solar-thermal collectors, to PVT collectors for which there is no formally established testing methodology at present. Three different types of PVT collector are tested, with a focus on the design parameters that affect their electrical and thermal performance during operation. Among other results, we show that a PVT collector suffers a 10% decrease in thermal efficiency when the electricity conversion is close to the maximum power point compared to open-circuit mode, and that a poor thermal contact between the PV laminate and the copper absorber can lead to a significant deterioration in thermal performance. The addition of a glass cover improves the thermal efficiency, but causes electrical performance losses that vary with the glass transmittance and the solar incidence angle. The reduction in electrical efficiency at large incidence angles is more significant than that due to elevated temperatures representative of water-heating applications. Dynamic performance is characterised by imposing a step change in irradiance in order to quantify the collector time constant and effective heat capacity. This paper demonstrates that PVT collectors are characterised by a slow thermal response in comparison to ordinary flat plate solar-thermal collectors, due to the additional thermal mass of the PV layer. A time constant of 8 min is measured for a commercial PVT module, compared to <2 min for a flat plate solar-thermal collector. It is also concluded that the use of a lumped, first order dynamic model to represent the thermal mass of the PVT collector is not appropriate under certain irradiation regimes and may lead to inaccurate predictions of the system performance. This paper outlines a procedure for the testing and characterisation of solar collectors, provides valuable steady-state and dynamic performance characterisation data for various PVT collector designs, and also provides a framework for the application of this data in a system model to provide annual performance predictions in a range of geographical settings.

The thermal emissivity of crystalline silicon photovoltaic (PV) solar cells plays a role in determining the operating temperature of a solar cell. To elucidate the physical origin of thermal emissivity, we have made an experimental measurement of the full radiative spectrum of the crystalline silicon (c-Si) solar cell, which includes both absorption in the ultraviolet to near-infrared range and emission in the mid-infrared. Using optical modelling, we have identified the origin of radiative emissivity in both encapsulated and unencapsulated solar cells. We find that both encapsulated and unencapsulated c-Si solar cells are good radiative emitters but achieve this through different effects. The emissivity of an unencapsulated c-Si solar cell is determined to be 75% in the MIR range, and is dominated by free-carrier emission in the highly doped emitter and back surface field layers; both effects are greatly augmented through the enhanced optical outcoupling arising from the front surface texture. An encapsulated glass-covered cell has an average emissivity around 90% on the MIR, and dips to 70% at 10µm and is dominated by the emissivity of the cover glass. These findings serve to illustrate the opportunity for optimising the emissivity of c-Si based collectors, either in conventional c-Si PV modules where high emissivity and low-temperature operation is desirable, or in hybrid PV-thermal collectors where low emissivity enables a higher thermal output to be achieved.

For hybrid photovoltaic-thermal collectors to become competitive with other types of solar energy converters, they must offer high performance at fluid outlet temperatures above 60 degrees C, as is required for space heating and domestic hot water provision, which together account for nearly 50% of heat demand. A roadmap is presented of the technological advances required to achieve this goal. Strategies for reducing convective, radiative and electrical losses at elevated temperature are discussed, and an experimental characterisation of a novel transparent low-emissivity coating for photovoltaic solar cells is presented. An experimentally-validated simulation formalism is used to project the performance of different combinations of loss-reduction strategies implemented together. Finally, a techno-economic analysis is performed to predict the price points at which the hybrid technologies along the roadmap become competitive with non-hybrid photovoltaic and solar thermal technologies. The most advanced hybrid technology along the roadmap employs an evacuated cavity, a transparent low-emissivity coating, and silicon heterojunction photovoltaic cells.

•PV-T driven air-conditioning systems can cover 60% of the domestic heating demand.•PV-T air-conditioning systems can cover up to 100% of the domestic cooling needs.•The importance of high resolution energy performance simulations has been demonstrated.•The LCOE of PV-T air-conditioning varies between 0.06 and 0.12€/kWh.

There have been a number of studies on the kinetics of the various B-O related defect transitions: degradation, regeneration, and destabilization. Some methods have monitored the inverse of minority carrier lifetime, 1/τ, at an injection level of Δn=1×10 15 /cm 3 . Other studies have used Voc, J01, or Jsc. This work summarizes some of these approaches and describes their sensitivity, behaviors, and validity. For an Arrhenius analysis, using the inverse of the minority carrier lifetime, 1/τ, at a fixed injection level is the only appropriate measure. However, this still leaves the question regarding which injection level to use. Commercially, the efficiency is of utmost importance. We show how the efficiency rate constant is different from the B-O defect rate constant due to the varying injection level of the maximum power point. An effective activation energy for the efficiency rate constant may be useful in predicting efficiency losses, but should not be used to describe the kinetics of the B-O reactions.

The main goal of this research is to design and assess a new heat recovery (absorber-exchanger) configuration for a hybrid PVT solar panel with a geometry and from a material that maximises heat transfer while reducing weight and cost. To this end, firstly, potential configurations and materials for the absorber-exchanger unit that have the potential to meet the aforementioned aim are considered. The selected configurations (plus a benchmark reference case) are then applied to the same PVT panel in order to compare their performance. Once these configurations are defined in detail, they are modelled in 3-D finite-element and multiphysics (FEM and CFD) software, specifically COMSOL. The objectives are to: i) maximize the heat transfer performance of the different absorber-exchanger configurations, ii) obtain characteristic curves that describe the steady-state performance of the PVT panel in each case, and iii) compare the different alternatives with the reference case. The results show that a polymeric flat-box configuration is a promising alternative to commercial PVT panels, being able to achieve an improvement in the thermal performance of the PVT unit compared to the reference case (up to 5.1% higher optical efficiency and up to 48.7% lower heat loss coefficient), while also lowering the weight (up to 12%) and investment costs (up to 22%) of the PVT panel.

In this paper, we examine integrated thermal energy storage (TES) solutions for a domestic-scale solar combined heat and power (S-CHP) system based on an organic Rankine cycle (ORC) engine and low-cost non-concentrating solar-thermal collectors. TES is a critical element and distinct advantage of solar-thermal systems. It can allow, depending on how it is implemented, improved matching to the end-user demands, improved load factors, higher average efficiencies and overall performance, as well as reduced component and system sizes and costs, especially in climates with high solar-irradiance variability. The operating temperature range of the TES solution must be compatible with the solar-collector array and with the ORC engine operation in order to maximise the overall performance of the system. Various combinations of phase change materials (PCMs) and solar collectors are compared and the S-CHP system's performance is simulated for selected months in the contrasting climates of Cyprus and the UK. The most important performance indicator of the ORC engine, i.e., net-power output, and the required TES volume are compared and discussed. The PCM-TES solutions that enable the best summer performance from an ORC engine sized for a nominal similar to 1-kWe output in combination with a 15-m(2) solar collector array result in diurnal volume requirements as low as similar to 100 L in Cyprus and 400-500 L in the UK. However, the required TES volume is strongly influenced by the choice of operational strategy for the system in matching the domestic load profiles. In a full-storage strategy in which electrical energy generation from the ORC engine is offset to match the week-day evening peak in demand, it is found that a similar to 20% higher total daily electrical output per unit storage volume can be achieved with a PCM compared to water as a sensible storage medium. The isothermal operation of the PCMs during phase-change allows for smaller diurnal storage temperature swings and higher energy conversion efficiencies from the solar collector array. These results are useful in informing the development of small-scale solar-thermal heat and power systems and of suitable integrated TES solutions for such applications. (C) 2017 The Authors. Published by Elsevier Ltd.

In this paper we present a dynamic model of a hybrid photovoltaic/thermal (PVT) collector with a sheet-and-tube thermal absorber. The model is used in order to evaluate the annual generation of electrical energy along with the provision of domestic hot-water (DHW) from the thermal energy output, by using real climate-data at high temporal resolution. The model considers the effect of a non-uniform temperature distribution on the surface of the solar cell on its electrical power output An unsteady 3-dimensional numerical model is developed to estimate the performance of such a collector. The model allows key design parameters of the PVT collector to vary so that the influence of each parameter on the system performance can be studied at steady state and at varying operating and atmospheric Conditions. A key parameter considered in this paper is the number of glass covers used in the PVT collector. The results show that while the thermal efficiency increases with the additional glazing, the electrical efficiency deteriorates due to the higher temperature of the fluid and increased optical losses, as expected. This paper also shows that the use of a dynamic model and of real climate-data at high resolution is of fundamental importance when evaluating the yearly performance of the system. The results of the dynamic simulation with 1-min input data show that the thermal output of the system is highly dependent on the choice of the control parameters (pump operation, differential thermostat controller, choice of flow rate etc.) in response to the varying weather conditions. The effect of the control parameters on the system's annual performance can be captured and understood only if a dynamic modelling approach is used. The paper also discusses the use of solar cells with modified optical properties (specifically, reduced absorptivity/emissivity) in the infrared spectrum, which would reduce the thermal losses of the PVT collector at the cost of only a small loss in electrical output when the selective coating is applied. (C) 2016 The Authors. Published by Elsevier Ltd.

The performance of hybrid photovoltaic-thermal systems can be improved using PV cells that are specially designed to generate both electricity and useful heat with maximum efficiency. Present systems, however, use standard PV cells that are only optimized for electrical performance. In this work, we have developed two cell-level components that will improve the thermal efficiency of PV-T collectors, with minimal loss of electrical efficiency. These are a spectrally-selective low- emissivity coating to reduce radiative thermal losses, and a nanotextured rear reflector to improve absorption of the near- infrared part of the solar spectrum for heat generation.

Grantham Briefing Papers analyse climate change and environmental research linked to work at Imperial College London, setting it in the context of national and international policy and the future research agenda. This paper and other Grantham publications are available from: www.imperial.ac.uk/grantham/publications