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For the SolSpaces project, researchers developed a heating concept with a seasonal sorption thermal storage system for the 43-square-metre energy-efficient research building built for the purpose (see image)
© ITW
Research projects revisited: SolSpaces
12.05.2016

Circulation within the storage tank is conducted in sections. When air enters into the inlet duct, it flows through a segment pair and exits from one of the four outlet ducts which run along the vertical storage tank edges.
© ITW

The rear side of the SolSpaces is designed without windows.
© ITW

The diagram demonstrates the solar thermal heat supply principle.
© ITW

Storing heat until winter

To be able to use summer heat for heating during the winter, hot water storage systems have to be very large. Small storage systems cool down too quickly. With this in mind, seasonal storage tanks have been developed almost exclusively for housing estates or very large consumers. Now researchers have created a new long-term storage system which is also suitable for single-family homes.

Since November 2013, a research house for energy-efficient living has been in operation in Stuttgart. As part of the SolSpaces project (BINE Information Service reported), scientists first equipped the 43-square metre research building with conventional heating technology. For a year, scientists at the Institute for Thermal Dynamics and Thermal Engineering (ITW) at the University of Stuttgart have been studying measurements of the building. The result: The annual heating requirements of the SolSpaces building, not taking heat recovery into account, amounted to around 3,000 kWh p.a. in an occupied state. When heat recovery is included, the value is about 30 per cent lower, at 2,000 kWh p.a. “The ratio between the surface and the volume of the research building is relatively high, and that leads to a comparatively high specific heating requirement per square metre,” explains Dr Henner Kerskes, head of the SolSpaces project. The absolute heating requirement for larger energy-efficient buildings is of a similar size. For this reason, the new heating concept can also be transferred to larger buildings.

The researchers at ITW developed a new solar heating system following the one-year monitoring period. The central components of the system include a sorption thermal storage system for seasonal thermal storage, an evacuated tube air collector with a collector area of 26 square metres on the roof of the research building, and a heat exchanger for heat recovery. The thermochemical heat storage system stores solar heat from the summer loss-free in order to heat the building during the winter. To date, hot water storage systems have generally been used. However, these require a great deal of space and are expensive. “For this reason, we developed a seasonal sorption thermal storage system which is also suitable for single-family homes,” the project leader adds.

Storing heat for long periods in small spaces

The principle of the sorption thermal storage system is based on adsorption and desorption. As an adsorbent material, the researchers use zeolite, which is known as a drying agent, and which is firmly embedded as a bead-like granulate in the storage tank. The natural air humidity of the ambient air has an adsorptive effect. The ambient air is guided through the zeolite granulate. Water molecules accumulate on the porous surface of the zeolite and adsorption heat is released. Conversely, the storage tank can be charged when solar heat drives out the water during the summer. Losses only occur during charging and discharging.

The particular feature of the sorption thermal storage system, which has a capacity of 4.3 cubic metres (see image top left) is the fact that it is subdivided into four square sections. These are each subdivided horizontally into six segments. 24 flat segments are created. In this way, section-by-section circulation within the storage tank is possible with only a low degree of pressure loss. Air enters the central inlet duct, flows through a segment pair and exits from the tank along the vertical storage tank edges towards the outlet duct. Each segment pair is assigned a shared air outlet opening to the outlet duct, enabling the segment pairs to be flowed through separately. “This makes flexible charging and discharging of the storage tank possible,” explains Mr. Kerskes.

The storage tank is charged at a temperature of 180 degrees Celsius. In order to achieve these high temperatures for solar energy systems, concentrating evacuated air collectors are used. For hot water storage systems, the typical storage density directly after charging is around 70 kWh/m3 and decreases continuously through heat losses. By contrast, for the loss-free zeolite storage tank, the scientists achieved an energy density of 163 kWh/m3. The entire storage capacity is therefore likely to be around 700 kWh.

Approximately 573 kWh were taken from the storage tank by the end of February. A precise analysis and evaluation can be conducted at the end of the 2015/2016 heating period when all measurement data for the heating period are available.

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Project management
Universität Stuttgart, ITW

Project partner
SchwörerHaus KG

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Summer sun against the winter cold
BINE-Projektinfo 02/2011