Pressurized storage tank The largest and most complex super-heated hot water storage tank has a volume of 46,000 m3 divided on two tanks. The maximal design temperature is 120 o C, and it is seperated from the grid by 10 bar. The energy content can be 3,700 MWh and it is installed at Avedøreværket CHP plant, which is a part of the integrated district heating system in Greater Copenhagen.
Figure 5. Vojens storage pit in operation May 2015
The water in the store is treated, however this is not sufficient to prevent corrosion. Therefore it is separated from the district heating water by a heat exchanger (difference 3°C), and the steel diffusers are protected like off-shore equipment. The Virtual Electricity storage and smart grids Some might argue that we need electricity storages, as district heating cannot store electricity – but in reality, it can. District heating with large thermal heat stores, like the ones in combination with large heat pumps and electrical boilers acts like a virtual battery. The point is that we use a lot of surplus electricity to load the heat storage in periods of low prices and avoid use of electricity in periods of high prices. This has the same impact on the power system as a battery – only it is much cheaper. In case of wind, the period of fluctuations has a length of typically a few days or weeks and therefore we need large storage volumes. Likewise, we can establish huge chilled water storage tanks and low temperature ground water storages for heating and cooling in combination with heat pumps in district cooling systems. Taking these advantages into account, the market share of district heating and cooling (DH&C) will increase as it makes DH&C a more competitive alternative to individual heat pumps and compressors in the urban planning and in the commercial market. By investing in DH&C instead of uncontrolled small heat pumps and chillers, we create smarter energy grids and a huge virtual electric battery. This opportunity for creating smarter grids will only be fully utilized if the building codes and standards not only focus on efficiency, but also promote integrated low-temperature heating and high-temperature cooling HVAC systems, which can be supplied from DH&C systems or building level ATES systems in case DH&C is not available.
Figure 3. Storage tank at Avedøre Copenhagen
Pressure less storage pit The largest storage pit has a volume of 200,000 m3. The maximal temperature is 90 o C. It can store 11,000 MWh. It is owned by the consumer owned district heating company in Vojens, Denmark. We have 4 pits of this type owned by small consumer co-operatives in operation in Denmark and several more are in the pipeline.
Figure 4. Vojens storage pit October 2014
This new technology is a combination of two known technologies - protected landfills and hot water storage tanks. The “only” new component is an insulated floating cover. It seems simple, but faces many practical challenges. Therefore the construction, developed by Ramboll and Vojens District Heating Company, is a further development of a smaller 10,000 m3 test plant in Marstal. The double-sealed bottom plastic liner is supplemented by a floating top liner covered by a 60 cm layer of insulating “ceramic leca nuts”(expanded clay), which again is covered by a top-liner. The final cover will be strong enough to carry a small car or some green vegetation. So we have also created the world largest water bed.
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