Photovoltaicthermal hybrids and concentrating elements

Johan Nilsson, Bengt Perers and Björn Karlsson 14.2.1 Co-generation of electricity and heat

A photovoltaic-thermal (PV/T) hybrid is, in principle, a cooled PV module where electric energy and heat is extracted simultaneously. A typical hybrid is shown in Figure 14.2.1, where polycrystalline silicon cells laminated on a conventional solar absorber with a copper tube are visible. The heat from the hybrid absorber shown in Figure 14.2.1 is collected by water running in the copper tube. Another option is to cool the cells with air and to use the hot air for ventilation or to heat water. Concentrators can be used to increase the irradiation on the hybrid, and this is discussed in more detail in section 14.2.4.

Source: Energy Research Centre of The Netherlands

Figure 14.2.1 Photovoltaic-thermal (PV/T) module with polycrystalline silicon cells taken apart to show the principal design

Source: Björn Karlsson

Figure 14.2.2 Spectral distribution of solar radiation and the internal quantum efficiency for a silicon solar cell

When the solar radiation impinges on the solar cell it gives a voltage of around 0.6 V between the front side and back side of the cell, and simultaneously heats up the cell and the fin. If the fin is cooled by feeding water through the copper tube, both electric energy and heat can be obtained. The mechanisms for transformation of radiation to heat and electricity in a solar cell are partly explained in Figure 14.2.2, which shows the spectral sensitivity of a solar cell.

A silicon PV cell absorbs all radiation for wavelengths below the band gap (1.1 ^m) and converts some of it to potential energy of the electrons, which can be extracted as electric power. Most of the radiation energy is, however, converted to heat. The cell is transparent for wave lengths above the band gap. This means that all of this radiation will be transmitted through the cell and absorbed in the absorber fin. For wavelengths below the band gap, the radiation is converted to electricity with varying efficiency. A commercial solar cell has an efficiency of 10 per cent to 15 per cent for transformation of radiation to electric energy, which means that 85 per cent to 90 per cent of the solar radiation is converted to heat. This heat is lost to the surroundings for a conventional PV module. In a hybrid, part of this heat is, instead, used for heating up the cooling medium, which is normally water or air.

14.2.2 Photovoltaic-thermal (PV/T) hybrids versus separate systems

The performance of a typical solar cell decreases with 0.4 per cent per centigrade of temperature increase. Correspondingly, the efficiency of a single crystalline silicon module decreases from typically 14 per cent at an operating temperature of 25°C to 12 per cent at an operating temperature of 60°C.

The performance of a solar collector also deteriorates with an increasing operating temperature since the losses to surroundings are proportional to the temperature difference between the absorber and the ambient. This means that the production of both heat and electricity is favoured by lowering the operating temperature. A minimum temperature of the heat is, however, generally required by the given application. The minimum temperature for pool heating is typically 25°C, for DHW 55°C, and for solar district heating 75°C. Another issue that complicates the problem is that the temperature of the cells in Figure 14.2.1 during high irradiance is 5°C to 10°C above the temperature of the cooling medium.

Table 14.2.1 shows the thermal performance of a standard flat-plate collector with a selective absorber in comparison with a collector with a hybrid absorber. The hybrid absorber has a high thermal emittance and, therefore, a higher U-value than the standard collector. The optical efficiency is usually also lower since the absorptance of the silicon cell is lower than the absorptance of the thermal absorber. When an electric load is connected to the PV cells, the optical efficiency is further decreased since part of the radiation is converted to electricity.

The cell temperature will be around 65° during operation in a DHW system with a hybrid collector. If a standard PV module is assumed to have an operating temperature of 40°, the annual electric output of the hybrid will be around 10 per cent lower than the output of the standard module. As a heat collector, the hybrid absorber exhibits a relatively high thermal emittance and this results in higher heat losses and, therefore, lower efficiency. The performance deterioration of the thermal output is also in the order of 10 per cent.

Compared to one PV module beside a thermal collector, a hybrid with the same number of cells and the same collector area will deliver approximately 10 per cent less electricity and 10 per cent less heat. The PV/T system will, however, only use half the mounting space of the side-by-side system, and this will balance the performance decrease. The fact that the two components are combined into one will also decrease the cost of installation. Another important benefit of the hybrid system is the uniformity in appearance. Solar collectors and PV modules installed beside each other will have different appearances, while PV/T modules all have the same appearance, and this will give a more uniform impression.

Table 14.2.1 Typical performance parameters of flat-plate collectors and flat hybrids

Optical efficiency n0 Heat loss factor F'U (W/m2K)

Collector with selective absorber 0.75 4.4

Hybrid collector without electric load 0.72 6.8

Hybrid collector with electric load 0.66 6.8

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