07/03/2012

Solar thermal technology in building

8993613-3d-illustration-of-men-installing-solar-panels.jpgAlthough the solar thermal technology is mature with competitive prices, it is not yet playing the important role it deserves in the reduction of buildings’ fossil energy consumption. The generally low architectural quality characterizing existing building integrations of solar thermal systems pinpoints the lack of design as one major reason for the low spread of the technology. As confirmed by the example of photovoltaics, an improvement of the architectural quality of building integrated systems can increase the use of a solar technology even more than price reductions and technical advances. As opposed to electricity produced by photovoltaics, heat is very sensitive to transportation losses. Therefore solar thermal collectors have to be mounted very near to the consumption place – i.e., on the building itself – making it even more urgent to correctly address the architectural integration issue.


The great potential of active solar thermal technologies to help reduce emission of CO2 gases in the building field is highlighted. The urgency to improve the integration quality of the BIST (building integrated solar thermal) systems to support the spread of these already efficient and cost effective technologies is emphasized. The various available technologies are presented and the most promising for building applications are identified as glazed and unglazed flat plates, as well as evacuated tubes. The interest of facade applications is discussed in terms of added exposed surfaces and energy production specificities.

Solar thermal technology in building

The world’s current energy demand is growing continuously, together with the related CO2 gas emissions in the atmosphere resulting from the use of non-renewable energies.

In such a context the role of renewable energies will be crucial to help reach, or at least get close to, the Kyoto protocol objectives demanding for a global reduction of greenhouse gas emissions of 5.2% compared to the year 1990 (corresponding to a cut of about 30% as opposed to the emission levels that would be expected without the protocol by 2010!).

Among renewable energies, solar energy is an enormous resource in comparison to fossil energies: the sun radiation energy reaching the earth in one hour is higher than the actual energy demand of the whole world for one year!

Within solar technologies, a crucial role is played by solar thermal systems (heating and cooling), This is a highly efficient, though simple and proven technology, with a payback time much shorter than its lifetime. Its efficiency is about four times higher than that of photovoltaics, with a cost per kWh of three to six times cheaper. Even if these last few years the market is growing fast (+34.9% worldwide and +62.5% in EU in 2008, but –9.6% in EU in 2009!) the solar thermal technology is really not playing the important role it deserves in the reduction of the fossil energy consumption of buildings.

The 35.9 million m2 of solar thermal (about 3.5 GWth power) installed in EU at the end of 2010 are still very far from the goals set by the White Paper on Renewables in 1997, asking for 100 million m2 by 2010 (i.e., 70 GWth).

One reason for the low spread of the solar thermal technology can certainly be found in the lack of appropriate promotion policies, an issue EU is finally facing by trying to set common targets and new rules1.
However, the cost effectiveness and simplicity of solar thermal systems indicate that this is not the sole reason for the general lack of interest for these technologies by building professionals. The generally low architectural quality characterizing existing building integrations of solar thermal systems pinpoints the lack of design as one main reason for the low spread of the technology.

As confirmed by the example of photovoltaics, the formal aspects related to the development of a solar technology also have to be carefully treated to make solar systems appealing to both users and building designers (Figs. 1.6 and 1.7). For solar thermal systems, building integration is crucial: whilst PV can be installed far from the consumption place (electricity can be transported and stored easily in the grid with very low losses), solar thermal energy, on the other hand, needs to be produced and stored near the consumption place, i.e., in the building itself.

The book of Maria Cristina Munari Probst intends to investigate the possible ways to enhance the architectural quality of building integrated solar thermal systems, and it focuses on integration in the facade, where the formal constraints are major.

Solar thermal technologies

Solar thermal energy can be collected in several ways and can be used for various building applications: space heating, domestic hot water production (DHW), and soon also for building cooling. It can be collected passively, through the transparent parts2 of the building envelope, storing the gains in the building mass itself [1.19]. These systems can only be used for space heating, and will not be further considered here as they are part of the standard knowledge of architects. It can also be collected actively on surfaces optimized for heat collection (solar absorbers) placed on the outside of the building envelope and transported by a medium either directly to the place of use, or to a storage tank to be used when needed. Among active systems, two main families can be identified according to the medium used for the heat transport: air collector systems and hydraulic collector systems.

Air systems are characterized by lower costs, but also lower efficiency, mainly due to a low air thermal capacity (0.32 Wh/K m3). Solar thermal gains are generally used immediately and without storage for pre-heating the fresh air needed for building ventilation (Fig. 1.8). The heat can be stored by forcing the air to circulate in a stones bed underneath the ground, or using the solar air as a cold source in an air/water heat pump; such applications can be quite expensive, and are rare. Like passive systems, air systems can only be used for space heating and will not be further considered.

Hydraulic systems, as opposed to air systems, allow an easy storage of solar gains and are suitable both for domestic hot water (DHW) production and space heating (soon also for cooling). As they are also cost effective, they are by far the most promising systems in relation to the goals set by the White Paper and are the main focus of this book (Figs. 1.9 and 1.10). Their medium consists mainly of water charged with glycol in variable percentages to avoid freezing according to the specific climate. The great thermal capacity of water (4.16 MJ/K m3, i.e., 1163 Wh/K m3) ensures a very good quality of heat exchange both with the absorber and the storage. The solar gains can then easily be stored in insulated water tanks and be used for domestic hot water (DHW) or/and space heating on demand. If used only for the space heating, solar gains can also be stored directly in the building mass, but their use in relation to the energy demand is less flexible.

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 Extracted from Architectural integration and design of solar thermal systems
written by Maria Cristina Munari Probst and Christian Roecker
Published by The EPFL Press

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