PVT hybrid types

Water or air are the common heat collection media in PV/T hybrids (Elazari, 1996; Hollick, 1998). Principal sketches of the two types are shown in Figure 14.2.3.

The hot water can be used for space heating, DHW or pool heating, as was discussed in previous sections. The cold water is pumped into the hybrid collector in the lower part of the figure and is led through the module and out the top, and the heat is collected in a storage tank. Figure 14.2.4 shows a PV/T water hybrid.

For the PV/T air system, the air can flow naturally on the back side of the PV cells, or it can be forced using a fan. Forced circulation makes the heat collection more efficient, but it is at the expense of a higher electricity demand due to the fan. The warm air can be used in ventilation for preheating of the incoming air, in which case the ventilation fan is used to circulate the air.

Source: Energy Research Centre of The Netherlands

Figure 14.2.3 Water cooled PV/T hybrid (left) and air-cooled PV/T hybrid (right)

Source: Energy Research Centre of The Netherlands

Figure 14.2.3 Water cooled PV/T hybrid (left) and air-cooled PV/T hybrid (right)

Source: Energy Research Centre of The Netherlands

Figure 14.2.4 PV twins: A PV/T water module

The main problem with an air-cooled PV system is to find a suitable need for the preheated air. The hot air is generally obtained during time periods of relatively high ambient temperatures, when the heating load is low. This effect will be even more pronounced in a well-insulated building with a requirement for heating during low ambient temperatures only, or in mild climates. This means that it is interesting to use the hot air for heating water via an air-to-water heat exchanger. The natural circulation of air that occurs when the PV module is heated during the day can also be used in natural ventilation in mild climates (Tripanagnostopoulos et al, 2002).

14.2.4 Concentrating elements for PV/T

The overall problem with the use of PV systems is the high costs for the modules. This makes it interesting to concentrate irradiation on the PV module, thereby minimizing the required PV area. This will also reduce the energy payback time of the system considerably. This is of interest since the use of PV cells significantly increases the energy payback time of a hybrid compared to a conventional solar collector.

Most conventional concentrating systems track the sun. Tracking systems are, however, not practically or economically suitable for building integration due to the complexity of the tracking system and the moving parts.

Non-tracking concentrators have been developed in the field of solar thermal collectors. One such family of concentrators is called compound parabolic concentrators (CPCs). A standard type CPC is illustrated in Figure 14.2.5. It is a trough system with parabolic mirrors and a flat absorber in the bottom collecting the irradiation.

When such concentrators are used in PV applications, there is a problem of elevated temperatures of the cells due to the high intensity of the irradiation. This problem is solved if the normal PV laminate is substituted by a PV/T absorber, which, in that case, will cool the cells effectively. A concentrator such as the CPC in Figure 14.2.5 will only accept light from a limited angle of incidence interval, indicated by 0a in the figure. This can be seen in the right part of the figure that shows that all light with an angle of incidence less than 0a is accepted in a full CPC. The laws of thermodynamics show that the concentration factor (how much the light can be concentrated) is determined by this interval of acceptance according to Equation 14.1. The interval of acceptance has thus to be chosen as an optimum for collecting as much irradiation as possible:

Source: Energy Research Centre of The Netherlands

Figure 14.2.4 PV twins: A PV/T water module

Source: Johan Nilsson

Figure 14.2.5 Geometry oof a compound parabolic collector (CPC) (left); the angular acceptance for a full and a truncated CPC (right)

A CPC with a high concentration ratio will be deep due to the parabolic shape of the mirrors. By truncating the mirrors, as is shown in Figure 14.2.5, the size can be reduced considerably without changing the concentration ratio much. The acceptance function of a truncated CPC is shown in the right part of the figure. As can be seen in the left part of the figure, the aperture of the truncated CPC is smaller, and less light will therefore be collected.

The concentrator in Figure 14.2.6 is required to be tilted around its east-west axes four to six times per year in order to keep the solar radiation within the acceptance angle of the concentrator. This type of concentrator is obviously not very well suited for building integration. It should be installed on the ground or on a flat roof. A roof installation can be seen in Figure 14.2.6.

Source: Bjorn Karlsson

Figure 14.2.6 The geometry oof a truncated standard CPC concentrator with a concentration factor of C = 4 and an acceptance angle oof q = 12°

14.2.5 Concentrating elements for building integration

Building integrated concentrating elements can be obtained through optical geometries that are strongly related to the geometry in Figure 14.2.5 (Mallick et al, 2004). Figure 14.2.7 shows a parabolic trough concentrator for roof integration. The hybrid absorber in the centre of the trough receives irradiation from both mirrors. The system is stationary and is intended to be installed with the absorber along the east-west axis. Due to the fact that the two parabolic mirrors are tilted at different angles, there is no need to move the trough at all during the year. This system has been designed for cold climates with low solar radiation during the winter. The back mirror reflects most of the light in the winter, spring and fall, and the front reflector reflects most of the light in the summer. The tilt and size of the mirrors have been designed to collect the maximum amount of annual irradiation.

Figure 14.2.7 Stationary asymmetric CPC concentrator (MaReCo) installed on a roof in Stockholm; the acceptance interval is 20° to 65°

Figure 14.2.7 Stationary asymmetric CPC concentrator (MaReCo) installed on a roof in Stockholm; the acceptance interval is 20° to 65°

Another building-integrated PV/T hybrid is the solar window. The window is shown in Figure 14.2.8. It consists of adjustable parabolic reflectors and a hybrid absorber. When the reflectors are in the open position shown in the figure, daylight will enter the room. No light is concentrated onto the

Source: Fieber (2005)

Figure 14.2.8 The solar window: The window is shown in open position, when daylight will enter the room; when the window is closed (the reflectors are tilted clockwise), the light is concentrated 2.45 times onto the hybrid absorber absorber; only light striking the absorber directly will generate heat and electricity. When the reflectors are closed, all of the light will be concentrated onto the absorber. The system has a concentration factor of 2.45.

The reflectors are insulated on the back side and this will lower the U-value of the window when the reflectors are in the closed position. The window is intended to be closed when it is dark outside and when it is not cloudy. It will then reduce the heat losses at night and concentrate the light as much as possible when there is enough sunlight available, while working as a shading device. It is not possible to concentrate the diffuse radiation on cloudy days, and the window will therefore be open to give as much daylight as possible.

The reflector of the solar window is well suited for building integration on a wall due to its dimensions: the depth of the system is small in comparison to its height. This makes it possible to manufacture wall elements that can be easily integrated within the façade. Figure 14.2.9 shows such a system integrated within a building in Aneby, Sweden (Adsten et al, 2005).

References

Adsten, M., Helgesson, A. and Karlsson, B. (2005) 'Evaluation of CPC-collector designs for stand-alone, roof or wall installation', Solar Energy, vol 79, no 6, pp638-647 Elazari, A. (1996) 'Multi-purpose solar energy conversion system', Solar Energy, vo. 57, no 3, pIX Fieber, A. (2005) Building Integration of Solar Energy - A Multifunctional Approach, Report EBD-T-05/3), Division of Energy and Building Design, Department of Construction and Architecture, Lund University, Lund, Sweden Hollick, J. C. (1998) 'Solar cogeneration panels', Renewable Energy, vol 15, pp195-200 Mallick, T. K., Eames, P C., Hyde, T. J. and Norton, B. (2004) 'The design and experimental characterization of an asymmetric compound parabolic photovoltaic concentrator for building façade integration in the UK', Solar Energy, vol 77, no 3, pp319-327 Tripanagnostopoulos, Y., Nousia, T., Souliotis, M. and Yianoulis, P (2002) 'Hybrid photovoltaic/thermal solar systems', Solar Energy, vol 72, no 3, pp217-234

Websites

Solarwall: ww.solarwall.com

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment