DEVELOPMENT OF AN OPTIMIZATION SYSTEM The recommendations of the Strategic Energy planning project Heat Plan III for Greater Copenhagen (2014), as well as the recent recommendations from the Danish Energy Commission’s report, highlight the potential ability of DH systems to be more flexible and become efficient integrators of fluctuating renewables (wind and PV). The recommendation for the Copenhagen area alone was to build heat storages in the magnitude of up to 10 times the storage capacity of the actual PTES in Høje Taastrup. A significant challenge for realizing such investments is to convince the investors that the storage will be operated in such a way that potential benefits can be attained in the real world. Currently, the operation of the Copenhagen DH system is operated by the central dispatch unit, Varmelast, which every day makes hourly based least cost plans for the operation of the production units and heat storages in the system. The plans are based on input from the different producers as well as forecasts for the heat demand and constraints in the heat transmission system. The optimization horizon of Varmelast is two days and it therefore does not take into account the value of the use of a storage beyond this time horizon. Therefore, it is necessary to develop an optimization system that can incorporate a longer planning horizon, for example, a week, and a different approach to optimization of the operation of a heat storage. The benefits of the PTES will be distributed between stakeholders. As the PTES will be integrated in the district heating system it will have an influence on all heat producers owned and operated by different companies (CHP, incineration plants, heat only boilers, geothermal plants and heat pumps.
The two main benefits of the PTES are: • that it can improve the flexibility of the DH system thus enabling 1) a better optimization of the production units in the electricity market, and 2) an increase of the heat production on cheaper units (CHP plants, geothermal energy and heat pumps), • decrease the production on more expensive units (natural gas and oil fired peak load boilers). In addition to saving expensive fuels, the PTES also reduces CO2-emissions by substituting fossil fuels with biomass and other renewable sources. Since the lifetime of the liners depends on temperature and oxidation, this will be a challenge to face in the FLEX_TES project. One of the possible liner suppliers has recently tested a liner at SP (Swedish test institute), and the Danish Technological Institute has also tested a liner, but in at shorter test period. The conclusion is that it is necessary to use a double liner solution to secure more than 20 years lifetime at 90°C. Also, the lifetime of the PE insulation will be a challenge since the melting point of the material used in Dronninglund is between 95°C and 100°C, meaning that the material will not stand 90°C in 20 years. But the supplier of the insulation in Dronninglund, is developing a new type of PEinsulation with a melting point at 120°C.
These two major challenges will be met when defining the demand specifications in the quotation of liners and insulation.
Since the storage function is to have the role as accumulation tank, on aweekly basis, for the entireDH system inCopenhagen, the in and outlet capacity has to be as large as possible. The inlet capacity will be 30 MW and the outlet capacity will be 30 MW, limited by the existing pipe connection to VEKS and the heat consumption in Høje Taastrup.
Fig. 5: Pumping station for the PTES
D I S T R I CT ENERGY - SUS TA I NAB L E C I T Y T RANS FORMAT I ON
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