Ra Rayleigh number

REF Reference case

S Area [m2]

STC Standard Test Conditions

T Temperature [°C]

U Internal energy [kJ/kg]

v Specific volume [m3 kg-1]

w Velocity [m/s]

Wcomp Heat pump compressor power

z Altitude [m]

Subscripts

ave average

cell Photovoltaic cell

conv,int internal convection

conv,ext external convection

El electric

in inlet section

Inn innovative

load Production

nc natural convection

out Outlet section

rad radiative

rec recovered

Ref reference

SH Super-heating

SC Sub-cooling

STC Standard Test Conditions

Th Thermal

tile tile

Introduction

The building energy demand has grown rapidly over the past few years。 This has led to the development of energy-efficient solutions, mainly renewable production systems, as photovoltaic or photovoltaic-thermal collectors。 Solar energy for instance can be easily exploited for air heating in residential applications to reduce the air conditioning consumption in winter period。

This study proposes the application of a heat pump combined with a solar heater。 In particular, the solar heater is made of the roof base and the tiles, leading to the so-called bare plate concept [1]。 This air heater concept is interesting for its simplicity and it does not add any costs beside the evaporator at the top of the roof。

Two different types of tiles will be considered: the first is a conventional square tile, while the second of the same size integrates a photovoltaic cell。 This second case fills into the Building Integrated Photovoltaic concept (BIPV)。 BIPV are often combined with heat recovery systems to reduce the photovoltaic cell temperature: lower cell temperature means higher conversion efficiency and lower cell degradation phenomena。

Most of the works available in literature on solar driven heat pump focus either on the heat pump or on the solar air heaters modelling [1–11]。 The present work covers both aspects with the same accuracy。

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When solar air heaters are investigated in BIPV configurations, the focus is typically on the heat transfer phenomena on the back of the PV module。 Different concepts have been proposed based on convective air flow either using an open loop system [1,2,4,12], or close loop water-based [13–15]。 Although air has a significantly lower heat transfer coefficient than water, it is widely used for the considered application because of (i) the simpler implementation, (ii) small channels are permitted, and (iii) no phase change issues occur。

Another aspect discussed in BIPV concepts is the system lay-out。 Pantic [16] proposed three different open-loop air BIPV/T roof systems。 They found that the addition of a 1。5m vertical glazed solar air collector to the unglazed BIPV/T system increased the thermal energy production in winter up to 50%。 The addition of a glazing over the PV panels was also investigated showing significant thermal energy production, but lowering the electricity production by 25%。 [17], through a full scale solar simulator built at Concordia University, investigated improved design of a BIPV/T system with multiple inlets and a vertical glazed solar air collector。 Simulation results indicated that the application of two inlets on a BIPV/T collector increased thermal efficiency by about 5%, while the vertical glazed solar air collector increases thermal efficiency by about 8%。 Kim [12] evaluated the electrical and thermal performance of an air type BIPV/T and a BIPV system applied to vertical building envelope。 From the simulation analysis, they evaluated the reduction of wall surface temperature compared to building, in which BIPV/T system is not installed and quantified the heating load reduction in 27%。 Ammari [18] investigated the effects of volume airflow rate, collector length and spacing between absorber and bottom plates on thermal performance of a modified single pass flat-plate solar air collector, provided of metal slats。 In all the above mentioned studies, the air flow was forced by a fan to increase the heat transfer coefficient。 As drawback, the fan consumes energy and increases system and maintenance costs。