Moving to the works focusing on heat pump aspects, [5] developed a simulation tool to predict the use of solar energy for air conditioning in Greece。 Results outlined the benefits of using solar energy to drive the heat pump instead of using it directly into the heated space。

Xu [8] outlined the advantages of solar assisted heat pumps against several competing technologies without discussing the heat recovery concept。

Finally, Ma [9] investigated the optimal refrigerant fluid among R22, R134a, R744 and CO2, showing that there is no clear advantage of any of these。

This paper aims at quantifying the passive heat recovery to drive a heat pump。 The roof tiles are made of Grès porcelain and can include a photovoltaic cell。 The heat recovery is based on natural convection under the tiles。 Since the temperature of the air at the top of the roof (i。e。 10 °C in winter) is usually well-below the typical range of residential heat demand, it will be exploited as low temperature source in an air-water heat pump improving its performances; a static finned tube evaporator for a direct heat exchange with the warm airflow is placed on the top of the roof。 The thermal power at the heat pump condenser supplies space heating and domestic hot water demand of a single family house, representative of Northern Italy。

This configuration is worth of investigation because it is simple and no additional costs are required for the air heating system。

A modeling tool is developed to determine the performances of the whole system from the passive heat recovery to the heat pump to outline advantages of this configuration with respect to reference heating concepts (i。e。 air-water heat pump using ambient temperature)。

The heat pump performance, the thermal energy produced, the electricity consumption and thermal energy produced are evaluated for the conventional and PV tiles as well as for different facade orientation for a typical year。

The results obtained are then expressed in terms of coefficient of performances (COP) and thermal energy produced by the heat pump, electricity consumption and thermal energy produced。

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Case study

This work is focused on the application of a tile for under-slating ventilation ducts。 The assessment is carried out for a detached house located in Northern Italy。 Accordingly, the typical temperatures and irradiations coupled with thermal loads are taken。 Main values are reported in Table 1 and Figure 1。

The detached house has a 100 m2 double pitch roof (see schematic of the roof in Figure 2)。 Both E-W and N-S orientation of the roof are considered in the analysis to outline differences and advantages/drawbacks。

The considered tile is made of Grès porcelain stoneware for its well-known frost and mechanical resistance。 Two different cases were considered differing by the integration of PV cells into the tile: one case with a PV module (named PV for simplicity) and the conventional tile without PV, (named Conv for simplicity)。 The PV tile integrates four monocrystalline PV cells with a peak power of 15。4 W。 Main characteristics of the considered tiles are summarized in Table 1。 The supporting structure of the tile is also shown in Figure 2。

The heating system includes an auxiliary air-water heat pump in series to the one driven by the heat recovered and a 1 m3 storage tank。 Radiant panel system is considered as terminals (35 °C inlet and 30 °C return temperatures)。

Methodology

The main objective of this study which is the application of tiles as solar heater to cover residential heat load can be split into two separate targets:

-Quantification of the thermal energy recovered from the covering surface;

-Assessment of the contribution of thermal recovery on the coverage of thermal loads。