Integration of wind and solar in the smart energy system Anders Dyrelund, Søren Møller Thomsen, Mogens Kjær Petersen and Niels Houbak Ramboll Denmark

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1 Integration of wind and solar in the smart energy system Anders Dyrelund, Søren Møller Thomsen, Mogens Kjær Petersen and Niels Houbak Ramboll Denmark It is a challenge for the power system to integrate the fluctuating wind and solar. The production does not match the consumption, and storing enough electricity in weeks and months corresponding to the fluctuations is impossible or extremely expensive. The obvious solution is to look at the whole energy system including power, district heating (DH), district cooling (DC), gas and the building installations. Already today some of these smart thermal energy systems can integrate a lot of surplus cheap electricity at low electricity prices and even generate electricity at high prices with CHP plants. The secret is that the District Heating and Cooling (DH&C) system in the transformation from fossil fuels to renewable energy can use a combination of CHP plants, heat pumps, electric boilers and large thermal storages benefitting from economy of scale. In total the systems acts like a battery a virtual battery. Moreover, carefully planned, this smart energy system can be extended at low cost and ensure that most heating and cooling in cities can be based on cost effective renewable energy. In many systems, the thermal storages already exists as the storages serve several other purposes, such as optimizing the operation of conventional CHP plants in the power market and levelling the fluctuations of the heat demand. The latest development is that large-scale solar heating has been the driver for development of huge cheap heat storage pits for storing solar heat from summer to winter. Thereby the storages are available at no cost for storing heat produced by electricity in at least 6 months. We can say that large-scale solar heating is a driver for developing large storages and more DH&C, which again is a driver for integrating fluctuating wind and solar. Once this obvious potential is utilized it will be time to convert the additional surplus electricity to gas and renewable fuels, which can be used by fast regulating CHP plants, being back-up for the wind and solar, thereby reducing the risk of black-out. In this paper we will give some examples on important symbiosis between the components in the smart energy system. 1

2 The heat storage tanks Heat storage tanks are becoming a natural partner of any CHP plant, which operates in the market. The typical design capacity should by experience at least correspond to 10 max-loadhours of the CHP plant. In figure 1 below we see a heat storage in Copenhagen designed for temperatures up to 120 o C. The actual storage capacity is 2,400 MWh at operation temperatures 105 o C supply and 50 o C return, corresponding to e.g. 300 MW constantly in 8 hours. Fig.1 Heat storage tanks at Avedøre CHP plant 2 x 24,000 m 3 In Denmark around 60 smaller DH companies have installed large-scale solar water heating (on the ground) to cover up to 20% of their annual heat production to supplement heat from gas CHP engines and boilers. Often a heat storage, which has been installed in combination with the CHP plant, has sufficient capacity to level the daily fluctuations of the solar heat. An example is Jægerspris District Heating in Denmark. Sometimes it has been profitable to increase the storage capacity in order to integrate more solar heat and to improve the operation of the CHP plant. An example is Silkeborg District Heating in Denmark, which owns a 108 MW gascc CHP plant. In order to integrate 20% of the heat production from a 156,000 m 2 solar heating plant it was profitable to increase the heat storage tank capacity from 2 x 16,000 m 3 to 4 x 16,000 m 3, (ref.1.). This larger storage also improves the performance of the CHP plant in the market. Besides the solar heating plant and the CHP plant share a heat pump, which improves the performance of both plants by reducing the return temperature. Chilled water storage Chilled water storages are even more important for the cooling system than heat storages are for the heating system. The main function of the storage is to level the large daily load fluctuations on the warmest days thereby reducing the cooling peak in the power system and reducing the need for installed cooling capacity. The second function is to optimize the production, typically by shifting production from day to night on normal days or to integrate electricity from solar PV, 2

3 in case there is an overflow of electricity in the local power grid. An example is Frederiksberg Forsyning, which has installed a large Chilled water tank in the new city district CarlsbergByen in Copenhagen in which 50 % of all floor area is expected to have a cooling demand. Integrating heating and cooling The symbiosis between DH and DC is an important element of the smart energy system. The two pipe systems go hand in hand and can support each other as a natural part of the urban infrastructure in most cities. The most important integration element is the co-generation of heating and cooling. The traditional cooling compressor, which waste the heat, can be upgrated or replaced by a heat pump, which can generate heat at a sufficient temperature for modern district heating systems, e.g. 75 o C. The addition cost of this replacement is only 15 % of the cost of a similar heat only heat pump. The same heat pump can generate heat and discharge the cooling the rest of the year, and its production can be optimized thanks to the storage tanks. Moreover, combined with ground source heating and cooling, the so-called ATES system, it can move the surplus heat from the cooling from summer to winter. Inter seasonal heat storage pits The first full-scale inter seasonal heat storage pit was developed by Marstal District Heating in order to increase the share of solar heat from 20 % to more than 50 %. The storage technology has been further developed, and heat storage pits up to 125,000 m 3 in Gram and 200,000 m 3 in Vojens (ref.2) have now been in operation two years on commercial conditions without any subsidies, only benefitting from saved tax on gas. In Figure 2 below we see the solar heating plant and heat storage pit in Gram. In the back ground we can see the old heat storage tank and the CHP plant. Fig. 2 Solar heating and heat storage pit in Gram Gram District Heating, which is one of 8 selected landmark DH&C cases in a newly published report by EU (ref.3) is a good example on a virtual battery. An electric boiler, a heat pump and a gas CHP engine enables the company to be very active in the power market benefitting from the fluctuating electricity prices, which reflects the weather conditions. The heat storage pit, which is 3

4 Heat production cost, Euro/MWh designed to store the solar heat, has available capacity to integrate other heat source from September to April. In new systems, the storage could be designed to store other competitive heat sources, also in the summer, e.g. heat from waste incinerators. In figure 3 below we see how a company like Gram could optimize the heat production taking into account the electricity price in case there was no taxes and the same market price was valid for all production plants Heat pump Elec. Boiler Gas CHP Gas boiler Electricity price, Euro/MWh Fig. 3 Heat production optimization based on electricity market prices without taxes The district heating company should plan the production weeks and months ahead taking into account the best available forecasts for electricity price and the solar heating as well as the available heat storage capacity. Assuming the company has estimated that it should be possible to generate the heat to meet the demand at a price not more than 20 Euro/MWh, the daily hourby hour operation strategy would be as follows: The electric boiler in operation at max load while the electricity price is less than 20 Euro/MWh The electric heat pump in operation at max load while the electricity price is less than 60 Euro/MWh The CHP plant in operation at max load while the electricity price is above 70 Euro/MWh The week ahead planning should be updated e.g. every week taking into account deviations from the forecast. In figure 4 we see how the plant based on this strategy could respond on the very fluctuating electricity prices we had in January This period it typical for the critical weather condition which stresses our energy system. First we had 10 days of strong wind and low electricity prices. The heat pump and even the electric boiler could run all the time only interrupted by a short period of high prices in which the CHP could take over. After this period we had a typical winter weather in Northern Europe. A stable high pressure, no wind and low outdoor temperature. The prices increased, the hydro power storage was apparently unloaded and the electricity prices increased dramatically in Denmark, Sweden and even Norway. In this period, there were several peaks in which the CHP could operate at maximum capacity and the heat pump could stop. 4

5 Electricity price Euro/MWh 50 0 Electricity consumption 100kWh/h Fig. 4 The optimal operation in a critical winter period This case illustrates the optimal way of operating the plant if we have a reasonable weather and electricity price forecast. In the Danish context it is not a simple case, as the taxation of electricity and the distribution tariffs not yet stimulate smart use of electricity. New DH&C systems In order to decarbonize the heating and cooling system there is a strong focus on wind and solar PV as the main sources. We cannot control the wind and the sun, but we can to some extend control the electricity consumption in the longer term. In the planning of the transformation from fossil fuel sources to renewables in existing and new urban developments we have a choice. We can use electricity in individual buildings or we can use electricity via DH&C, in which the consumption can be adjusted to the prices and integrate the fluctuating sources. In our heat planning models in Denmark we can see that the most cost effective way to replace the fossil natural gas with surplus wind and electricity is to establish DH&C with storages supplemented by a minor share of heat from gas fuelled fast regulating CHP plants and boilers. This gas could in the longer term be based on renewable energy and stored in the natural gas storages. This transformation will take long time in Denmark and even longer time in countries with little market share of DH&C. Therefore it is of interest for all countries to look at new urban developments. We have in a strategic energy plan analysed the supply of heating and cooling to a new urban development, Favrholm in the town Hillerød in Denmark. The first stage is a new hospital next to a new station. Therefore the district will develop into a typical local town with offices and institutions close to the station and less densely residential buildings in the out skirts. The total heated floor area will be around 600,000 m 2 of which a bit more than half is expected to have a cooling demand too. After a selection of several alternatives we have compared two main alternatives: 5

6 Alternative 1, the base line. An individual solution in which individual heat pumps and chillers deliver heating and cooling to each building without any storage, which normal best practise for new buildings without DH&C. The individual system has little possibility to respond on the fluctuating electricity prices as thermal storages are relatively expensive at the building level and takes up space Alternative 2. An integrated DH&C system with ground source cooling (ATES) supplying all heating and cooling demand in the district. The integrated system can benefit from responding on the fluctuating electricity prices and thus form a virtual battery compared to the individual solution. The figure 5 below shows the main characteristics of the integrated DH&C system. Optimized DH&C to the district DH DC Length og network and branch lines km DH storage tank, rough estimate m DC storage tank m Capacity demand to network MW 12,0 11,0 Capacity leveling of storage MW 3,0 Ground source cooling MW 3,0 Gas boiler for peak MW 5,0 Total installed heat pump for DH&C MW 7,0 5,0 Total installed capacity MW 12,0 11,0 Necessary electric capacity MW 2 Total COP for cogen of DH&C MW/MW (7+5)/2 = 6 Fig. 5 DH&C system in the new city district As all buildings are new we can benefit from more efficient building installations following the requirements of the building code. The design temperatures are 60/40 for heating and 10/20 for cooling. This will increase the performance of the heat pumps for combined heating and cooling. We notice that the COP for co-generation of heating and cooling is 6 as both the heating and the cooling is fully utilized. Figure 6 below summarizes the total investments in the individual solution and the integrated DH&C solution. The total investments are almost the same in the two alternatives. In other words, the investment cost of this virtual electricity storage is zero. We only need good urban planning and co-operation among the consumers and the city to establish it. Investment in base line Heating Cooling Total Individual heat pumps / schillers mio.euro Investment in DH&C system DH DC DH&C DH&C networks mio.euro 20,0 7,9 27,9 DH&C storages mio.euro 1,6 0,8 2,4 DH&C boiler / ground source cooling mio.euro 0,7 1,1 1,7 DH&C heat pump for DH&C mio.euro 5,5 5,5 Total DH&C mio.euro 37 Fig 6. The total investments in the individual base line and the DH&C alternative 6

7 References: Ref.1 Ref.2 Ref.3 7