Water storage technology

Storing heat in water serves to bridge sunless periods in the case of solar hot water and combined heating systems, to increase the system efficiency in combination with cogeneration systems, and to shave the peak in electricity demand and improve the efficiency of electricity supply in the case of an electrically heated hot water tank.

Water tank storage technology is mature and reliable. Sensible heat storage in water is still unbeaten in terms of simplicity and cost. In refined systems, the inlet/outlet heights in the tank can vary according to supply and storage temperatures. Thermally stratified water tanks can improve the annual system efficiency by 20 per cent or more. Figure 13.1.1 illustrates variations of preserving the stratification effect to maximize storage efficiency. For combined space and water heating, multiple tanks are a good solution. There could, for example, be a short-term, mid-term and long-term tank.

The storage need in a solar system is often determined by the ratio of the maximum to minimum monthly solar radiation. Figure 13.1.2 gives this data for different latitudes. When the maximum-minimum ratio is less than 5, even wintertime solar energy may be enough to provide the heat load, whereas values higher than 10 mean such a large fluctuation that a seasonal storage or

Klagenfurt University
Source: Gerhard Faninger, University of Klagenfurt

Figure 13.1.1 Types of water storage systems to achieve stratification

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.1.2 Solar radiation by latitude, including minima, maxima and their ratios

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.1.2 Solar radiation by latitude, including minima, maxima and their ratios backup system is necessary. In high latitude Northern Europe, the winter solar radiation falls under the utilization limit.

Water heat storage can be categorized by the duration of the storage.

Short-term storage for solar hot water systems typically has a storage capacity between 1.5 and 2.0 times the daily hot water demand. Even with short-term storage, generous insulation of the tank is essential (see Chapter 12, 12.2 'Active solar heating: Water').

Mid-term storage for solar-combined heating systems and solar-supported district heating should cover the heat demand for three to five days. For detached and row single family low-energy houses, a storage volume of about 800 to 1500 litres will be suitable.

Seasonal storage is one means of achieving a high annual share of solar heat in northern latitudes. A realistic target is to provide a heat capacity of six months in existing housing or four months in highperformance housing (which have a shorter heating season).

Solar heating plants with seasonal storage may take a decentralized and a centralized approach. In a decentralized approach, the storage and collectors are placed within the individual houses as in an ordinary active solar heating system but of a larger size. In the centralized concepts, solar heat is collected in one storage unit from which the heat is distributed to many houses, as illustrated in Figure 13.1.3. The advantage of scale is that the relative heat losses and tank costs decrease with the decreasing tank surface-to-storage volume ratio of ever larger tanks. The relative heat losses are proportional to the surface/volume, or, V2/3/V = V-1/3. Therefore, as the relative losses ^ 0.

Such a system may be considered for low energy housing. For high-performance housing at the level of the Passivhaus standard, the costs and heat losses of the pipe network are proportionately very high for the absolute amount of heat required. A further advantage of a centralized system is the reduced unit costs when large numbers of collectors are purchased.

Figure 13.1.4 illustrates different large-scale sensible heat storage concepts. Concepts such as earth pits or rock caverns are large water reservoirs built into the ground. Aquifer storage employs the storage capacity of water-mixed ground. Aquifer storage is very simple and needs only a few wells to operate. Vertical pipes may be laid into the ground, enabling the use of the thermal capacity of the ground. Ground heat storage may also be employed effectively through heat pumps yielding a larger DT. The most frequently used 'seasonal' thermal storage technology, which makes use of the underground, is aquifer thermal energy storage. This technology uses a natural underground layer (for

CENTRAL SOLAR HEATING PLANT WITH SEASONAL STORAGE

Housing estates h Lyckebo/S weden

1(1,111 m3

water storage

4,321 m3 collector area

351 row bouses

4,321 m3 collector area

351 row bouses

Centralised solar thermal storage

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.1.3 An example of seasonal thermal storage example, sand, sandstone or chalk layer) as a storage medium for the temporary storage of heat or cold. The transfer of thermal energy is realized by extracting groundwater from the layer and by reinjecting it at the modified temperature level at a separate location nearby. A major condition for the application of this technology is the availability of a suitable geological formation.

Other technologies for underground thermal energy storage are borehole storage, cavern storage and pit storage. Pit storages are mainly used for offices and housing estates. Ground heat exchangers are also frequently used in combination with heat pumps, where the ground heat exchanger extracts low temperature heat from the soil. Large underground water storage (for example, cavern storage and pit storage) is technically feasible, but its application is still limited because of the high level of investment required.

Since solar thermal systems with seasonal storage are always site dependent, the design has to account for the local conditions. Detailed simulations and systematic variation of design parameters are necessary for the design and the analysis of the overall performance and economics.

Already, in some special cases, seasonal storage solar heating may be found economically justified; but this conclusion is not yet generally valid for other sites and applications. The practical possibilities for long-term storage would be dramatically improved with higher storage capacities within latent and chemical storage concepts (see Section 13.2 'Latent heat storage'). 1. Partly insulated earth pit 2. Rock cavern 3. Vertical pipes in ground

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.1.4 Concepts for seasonal thermal storage

Source: Gerhard Faninger, University of Klagenfurt

Figure 13.1.4 Concepts for seasonal thermal storage

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