Cogenerative PV Thermal Modules of Different Design for Autonomous Heat and Electricity Supply

Introduction

The energy of the Sun is one of the most common and most widely used sources of energy. The amount of energy coming from the Sun to the Earth exceeds the global consumption and for many years remains an inexhaustible source of energy for mankind. The scale of this energy source is very well illustrated by the figure 1.

Figure 1.

Annual world energy consumption in comparison with the solar energy amount coming to Earth throughout the year

Source: Fortov & Popel, 2011

Solar power is used in practice for obtaining thermal energy in the form of hot water through solar water heating collectors and for electricity generation through photovoltaic converter (solar cells) which switched in modules and batteries.

Photovoltaic (PV) System. Solar PV generation based on photovoltaic transformation of solar radiation into electricity. Due to efficient improvement of PV technology it is used as for electricity production at big solar power plants as well in small solar power installation for local energy supply of stand-alone remote objects. The last one is the most interesting area from the point of view of this book. PV generation is preferred worldwide as Distributed Energy Resources (DERs) now. The major advantages of a PV system are the sustainable nature of solar energy, positive environmental impact, longer life time and silent operation.

Solar collectors. The thermal energy in the form of hot water generates in the solar collectors of various types which the most widely used are flat-plate collectors.

Widespread liquid flat solar collector is a heat-absorbing panel (absorber), which consists of channels (tubes) for circulating the coolant. Above the absorber (with a certain gap) is located the transparent and insulating layer. The whole structure is placed in a housing, bottom and side parts of which are equipped with insulating material.

The principle of operation of the solar collector is based on the ability of glass to pass short-wave solar radiation and to hold long-wave radiation of the heated surface (absorber) – a phenomenon called the “greenhouse effect.” As a result of this selective transmittance of solar radiation absorbed by the panel absorbers will be emission of the long-wave radiation. And, thanks to the ability of glass to hold long-wave radiation, there is a significant increase in temperature inside the space confined by glass.

Application field of flat solar collectors is quite extensive. They are used for example in heating systems of residential and industrial buildings, hot water systems, as well as in the number of special energy plants.

For integrated power supply of the above objects, as a rule, the system is installed consisting of collectors as well as a set of power solar modules.

Attempts were made to implement these two processes in a single device later.

Background

The principle of formation of a single device, providing at the same time electrical and thermal energy is illustrated at the figure 2.

Figure 2.

Principle of formation of the photo-electric thermal module. 1 - PV module; 2 - solar collector; 3 - PV Thermal module

In this case, the PV Thermal (PVT) module creation is provided by placing solar cells on the heat absorbing surface of the flat solar collector. PV panel has different efficiency which depends on the type of used solar cells which converts solar radiation into electricity and the remaining solar energy converts into heat. The absorber in this design performs a dual function. First, it cools the PV panel, removing the excess energy that is not involved in the generation of electricity, thereby increasing its efficiency, and secondly, produces heat energy. Even in the case of use a flat collector it is necessary to solve some technological problems in order to the PVT module ha a high efficiency. The issue of creating a design of PVT module with higher efficiency in details will be considered below.