Operating a smart energy system As renewable and efficient energy is of low quality and fluctuating, tariffs and contracts should take into account time and quality as well as the cost structure. DH tariffs should reflect that. Smart electric grid tariffs should also encourage smart electricity consumption, which can disconnect at any time for as long as needed. Thermal storages Hot water storage tanks are now a proven technology: • Optimizing heat production from all large CHP plants and other sources. • Storing large-scale solar heat production from day to night. • Maintaining the pressure in the DH grid and storing make-up water. Cold water storage is important in a DC system: • The storage levels out the daily fluctuation on the warmest days and saves capacity • The storage helps to optimize the production other days, e.g. producing the cold during night hours. Short-term storage tanks are commonly known, but seasonal heat storage pits are a new innovative technology developed in Denmark as a spin-off from large-scale solar heating systems. Symbiosis of DH&C DC has several benefits in symbiosis with DH. One of them is co-generation of DH and DC directly or via an aquifer thermal energy storage facility.
We look at a small community like Gram, see e.g. new JRC report from EU. There is a large-scale solar heating plant and a heat storage pit to generate 50% of all heat, but from September to May there is available “free” capacity in the storage. An electric boiler, a heat pump and a gas engine can respond to electricity prices. To make it simple, we assume that the price of electricity is Nordpool hourly market prices to both the individual consumers and the DH system without taxes. Likewise, we assume that the gas price is EUR40 /MWh. The variable heat production cost can be seen below.
Figure 4 Variable heat production cost
The company can plan optimal production based on forecasts for the weather and the electricity price.
In Fig. 5 we can see how the electricity consumption responds to the market price. The consumption is 6 MW at low prices and the production (or "negative consumption") is 4.5 MW at very high prices.
Figure 3 ATES system
This is a five-birds-in-one solution: • Seasonal chilled water storage down to 4-10°C. • Seasonal hot water storage, 10-20°C. • The heat pump is cooling peak capacity in summer. • The heat pump cools ground water in winter. • The heat pump can produce more heat and discharge cooling whenever profitable Case 1. Existing DH system Below, the dynamics of the virtual electricity storage is illustrated with a simple case. In January 2016 , we had 10 days with strong wind followed by 10 days of low temperatures and no wind.
Figure 5 Demand response of DH
Comparing the electricity consumption/production of the DH system with the alternative constant consumption of the individual heat pumps illustrates how the DH system acts like a virtual battery. Fig. 6 shows the heat production hour by hour and the energy content of the heat storage.
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