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Flat plate or evacuated tube solar heating collectors are a well-established technology for individual buildings, especially at the domestic scale. The next development is likely to w

Figure 2.8 Mechanical circulation solar thermal system with back-up boiler (courtesy of Renewable Energy World, March-April 2004)

w cold water

Figure 2.8 Mechanical circulation solar thermal system with back-up boiler (courtesy of Renewable Energy World, March-April 2004)

be the scaling up of this technology to help meet heating requirements at district level. According to Dirk Mangold of the University of Stuttgart, 'Central solar heating plants offer one of the most economic ways of providing thermal solar energy to housing estates for domestic hot water and room heating. Over 50% of the fossil-fuel demand of an ordinary district heating plant can be replaced by solar energy when seasonal heat storage is included in the plant.'4

District solar heating plants consist of a network of solar collectors which may be sited on ground level rigs next to a central heating substation. In urban situations it is more likely that solar collectors will be mounted on roofs. In Germany and Sweden there are available complete solar roof modules for construction purposes, complete with rafters and insulation. The flat plate collectors replace the tiles. Compared with a conventional roof the additional cost is €150-250/m2 (excluding VAT) of flat plate area including all pipework and installation cost.

There are two basic systems of district solar heating: seasonal storage and diurnal storage. Central solar heating plants for seasonal storage (CHSPSS)

Banking heat in summer to meet winter expenditure is the principle behind seasonal storage. There are four principal storage technologies.

1. Aquifer heat storage: Naturally occurring aquifers are charged with heat via wells during the warming season. In winter the system goes into reverse and the warmth distributed through the district network (see Fig. 2.9).

2. Gravel/water heat storage: A pit with a watertight plastic liner is filled with a gravel/water mix as the storage medium. The storage container is insulated at the sides and top, and base for small installations. Heat is fed into and drawn out of the storage tank both directly and indirectly (see Fig. 2.10(a)).

3. Hot-water storage: This comprises a steel or concrete insulated tank built partly or wholly into the ground (see Fig. 2.10(b)).

4 Duct heat storage: Heat is stored in water-saturated soil. U-pipes are placed in vertical boreholes which are insulated near the surface. Heat is fed into and out of the ground via the U-pipes. Storage temperature can reach 85°C (Fig 2.10(c)).

Renewable Energy Fraction Total

Central solar plants for seasonal storage aim at a solar fraction of at least 50% of the total heat demand for space heating and domestic hot water for a housing estate of at least 100 apartments. The solar fraction is that part of the total annual energy demand which is met by solar energy.

In all these installations it is necessary to receive authorization from the relevant water authority.

By 2003 Europe had 45 MW of installed thermal power from solar collector areas of over 500 m2. The ten largest installations in Europe are in Denmark, Sweden, Germany and the Netherlands, mostly serving housing complexes. Germany's first solar-assisted district

Figure 2.11


Diagram of CSHPSS system, Friedrichshafen (courtesy of Renewable Energy

Figure 2.11


Diagram of CSHPSS system, Friedrichshafen (courtesy of Renewable Energy heating projects, launched as part of a government research project 'Solarthermie 2000', were at Ravensburg and Neckarsulm. These have already proved valuable test beds for subsequent schemes.

One of the largest projects is at Friedrichshafen, and can serve to illustrate the system (see Fig. 2.11). The heat from 5600 m2 of solar collectors on the roofs of eight housing blocks containing 570 apartments is transported to a central heating unit or substation. It is then distributed to the apartments as required. The heated living area amounts to 39,500 m2.

Surplus summer heat is directed to the seasonal heat store which, in this case, is of the hot water variety, capable of storing 12,000 m3. The scale of this storage facility is indicated by Fig. 2.12.

The heat delivery of the system amounts to 1915 MWh/year and the solar fraction is 47%. The month-by-month ratio between solar- and fossil-based energy indicates that from April to November inclusive, solar energy accounts for almost total demand being principally domestic hot water (see Fig. 2.13).

The Neckarsulm project is smaller, serving six multi-family homes, a school and a commercial centre, amounting to a heating area of 20,000 m2. The roof-mounted solar collector area comes to 27,000 m2 to satisfy a heat demand of 1663 MWh/year. In this instance the

Figure 2.12 Seasonal storage tank under construction, Friedrichshafen (courtesy of Renewable Energy World)

2 400

J 200

"05 Q

100 0

Figure 2.13 Monthly ratio of solar- to fossil-based energy at Friedrichshafen (courtesy of Renewable Energy World)

solar fraction is 50%. The storage facility is a duct heat storage tank with a capacity of 20,000 m3 (see Fig. 2.14).

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