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Utveckling av Byggnadsintegrerade Solenergisystem för Energieffektiva Hus

Author

Summary, in English

A PV/T collector has been developed and evaluated at the department of Energy and Building Design at the Technical University of Lund, LTH in Sweden. The PV/T, a “solar window” made of PV cells laminated on solar absorbers is placed in a window behind the glazing. The solar window is built into a single family house, Solgården, in Älvkarleö outside Gävle in the eastern part of Sweden. To reduce the costs of the solar electricity reflectors have been placed in the construction to focus light onto the solar cells. In this way expensive solar cells can be replaced by considerably cheaper reflector material. The tiltable reflectors give the user a possibility to control the amount of sunlight being let into the building. The reflectors can also be used to reduce the thermal losses through the window.

A calculation model for electricity and hot water production was created. The simulation program, in Excel, can perform yearly energy simulations where different effects such as shading of the cells or the glazing effects can be included or excluded. The simulation can be run with the reflectors in an active, up right, position or with the reflectors in a passive, horizontal, position. The simulation programme was calibrated against a prototype window placed in Lund in the south of Sweden and against the solar window in Solgården Älvkarleö. The calculation model serves as a basis for the module written for the simulation program TRNSYS. A “TRNSYS-deck” was built and calibrated for the building Solgården. The energy balance for the system shows that the control of the reflectors is of great importance. A good control strategy for the reflectors can lower the annual energy demand and improve the indoor climate. A control mechanism that incorporates the indoor and outdoor temperatures can for instance decide to open the reflectors during summer nights to cool the building.

Yearly simulations of the energy balance for a house with the solar window was compared to simulations where the 16 m2 solar window was replaced with a 8 m2 normal window. The results show the annual amount of auxiliary is lower with a developed solar window compared to the normal window case. The developed solar window has considerably lower U-values than the existing solar window. A simulation was run to investigate the over heating of the building using different control strategies for the reflectors. The simulation was also run for the normal window. The results show that the reflectors of the solar window work fine as a sun shade, keeping the overheating to a minimum. A normal window has to be supplemented with a sun shade of some kind to keep the over heating during the summer on an acceptable level.

An Improvement of the solar window that can be of great importance is to add some sort of low-e coating on the glazing to reduce the U-value even further. However, it must be taken into account that the transmittance of the glazing will be affected in a negative way if low-e coating is added. Lower transmission means lower electricity and hot water production. Other improvements that can be made are better insulation of the reflector and the absorber. The most important way to improve the annual electricity production is to use better solar cells. Today cells of standard quality with an efficiency of about 16-17% can be obtained. The electric system at Solgården is built up around a battery bank that can be charged from the solar window, the Stirling generator or from the grid. To have continuous power for some time during a black out and also the relatively low fuse that is needed for the house are two advantages with this system. Simulations show that the day to day storage of electricity in the batteries evens out the power peaks from the load and the electric production, and that further storage is of limited use. Simulations show that the production of electric energy from a combination of solar cells and a Stirling generator is in good agreement with the load during the full year.

Solgårdens two parallel electric systems, 230 V and 12 V, has many advantages such as electric security and low energy consumption. To save electric energy is of extra interest when solar cells are used as an energy source. Due to the high price for solar cells large energy saving investments ought to be made before a solar cell installation is economically interesting. To move the energy use from electricity to hot water use is profitable for a stand alone house since solar heating and bio fuel is considerably cheaper compared to solar electricity. Tumble driers, washing machines and dishwashers can for instance use a circulating hot water system instead of electricity in order to minimize the electric use. Normally the energy balance between produced electric energy and produced thermal energy is in better agreement if hot water is used instead of electric energy. Other important investments to make the electric usage more efficient are to use low energy light bulbs and modern kitchen appliances, pumps, fans and computers. This can save as much as 80% of the electricity compared to old standard techniques.

Solgården is in practice working with the main fuses of the sizes of 3-4 A only. This can be accomplished because of the battery bank and since no equipment with a high current demand is connected to the power system.

A high slope of the PV-modules has the advantage that the power during the summer is decreased. On the other hand a module tilt of 45° is to prefer both from an energy and a power view. A 45° tilted module gives a higher annual output in kWh per monitored kWp than a vertical module.

Publishing year

2008

Language

Swedish

Document type

Report

Publisher

Elforsk

Topic

  • Building Technologies

Keywords

  • PV/T
  • solar window
  • solfångarhybrid
  • collector
  • solfönster

Status

Published

Project

  • Climate Initiative