VEKS (municipality-owned heat transmission company) and HTF (consumer-owned heat distribution company) have implemented a Pit Thermal Energy Storage (PTES) in Høje Taastrup to provide flexibility to the electricity production system and the heat production system in Copenhagen.
The project was developed 2017-2018 and implemented 2019-2022. During implementation material tests were carried out and new implementation methods were developed.
Size of the pit storage is 70,000 m3 . The storage medium is water. In- and outlet capacity is 30 MW and storage contents are 3,300 MWh.
The project has several innovative elements
- It is the first PTES of its kind operating with flexibility and a constant top temperature of app. 90 oC.
- A new business model had to be developed in order to integrate the storage in the heat and electricity production system in Copenhagen.
- A recently developed PP-membrane has been employed. It is used for tightening bottom and sides and for the floating membrane underneath the lid. Implementation methods for membranes had to be modified as the membrane is vulnerable to low temperatures.
- In- and outlet system as well as the lid have innovative elements
The storage provide services for four CHP plants and three waste-to-energy plants. Optimization of electricity production (sector coupling) and saved peak production result in 27.4 TJ saved fuel/year and a total CO2-reduction of 6,200 tons/year.
The total investment is 10.7 Mio €. Simple payback period is 12 years and IRR is 7.5%.
The PTES was commissioned end of December 2022 and in commercial operation from February 15th 2023.
The History
The scenario analysis in the ”Heating plan greater Copenhagen” developed by the transmission companies CTR, HOFOR and VEKS in 2014 pointed out, that if CO2 neutrality in district heating in Copenhagen was to be obtained, large-scale thermal storages would have environmental and economic benefits in an energy system with a huge fraction of electricity from wind power. The reason for this being the ability of thermal storages to store district heating when it is cheap to produce and hence optimize the total electricity and heating production system in Copenhagen.
Follow up calculations showed that a thermal storage of 70,000 m3 could be economically feasibly implemented in the existing production system. HTF found a suitable area for the storage and HTF and VEKS decided to start the investigations and designs relevant to the project. It was decided to implement a Pit Thermal Energy Storage (PTES) because this kind of storage already existed in Denmark and because investment costs were 25-30% of the costs for steel tanks of similar sizes. Prices for excavation, in-and outlet system, membrane work, lid construction and the system for connection were found during 2017 and 2018. The Danish Energy Agency (EUDP) was also asked for subsidies through the funding program of EUDP because the project is innovative. Funding of 17% of the total costs (1.8 Mio. €) was agreed.
HTF and VEKS made the final investment decision at the end of April 2019, when the final permissions were given by the authorities.
Excavation for the PTES started in April 2020. Polymer membranes were implemented during summer, and in- and outlet was constructed in autumn 2020. After that water filling started, but during winter, the polymer membrane broke and could not be repaired. A new membrane was implemented from July 2021 until autumn 2021 and water filling took place during winter. From April 2022 welding and implementation of the floating membrane started and lid construction took place from July 2022 till November 2022. Commissioning took place during December 2022.
After tests of functions and running-in commercial handover took place February 15th 2023.
Design of the storage
The PTES in Høje Taastrup was designed with soil balance. I.e. excavated soil has been used as banks round the storage. Because the area is limited between a highway and a huge pipe of drinking water, the shape was forced to be as shown in Fig 1
Fig 1. Longitudinal section of the PTES
The function
The PTES is charged from VEKS´ transmission line and discharged into HTF’s distribution network. Charging and discharging capacity is 30 MW and the storage capacity is 3,300 MWh. VEKS´ transmission line is connected to the total transmission system in Copenhagen. The PTES provides services for four CHP plants of a total of 2,050 MWheat owned by Ørsted, HOFOR and VEKS and three waste-to-energy plants of a total of 400 MWheat. Further to that, the DH production system has 1,900 MW reserve and peak load plants. In Fig 2 the DH (district heating) in Copenhagen and connection pipes are illustrated.
Fig 2, – DH in Copenhagen and transmission pipes to the PTES. Yellow is VEKS´ transmission pipes, blue is HTF´s distribution pipes and red is charging and discharging pipes between the heat exchanger building and PTES.
The pit storage was constructed to optimize the heating and electricity production in Copenhagen in three ways
- Optimization of CHP production according to the electricity market (sector coupling)
- Reduction of peak load production on natural gas boilers
- Extension of the heat production from the waste-to-energy plants during the summer period
Innovative parts
Until now PTES has been implemented in connection with solar thermal plants where the PTES is heated in summer and cooled in winter. In Høje Taastrup the PTES is implemented to provide flexibility to the electricity and heat production. It means that the PTES will be continuously charged and discharged and that the top temperature in the water will be 85-90 oC constantly. This function and temperatures are new for PTES.
High Density Polyethylene (HDPE) membranes have been used for watertightening PTES-es connected to solar thermal, but HDPE will according to the supplier brake down after app. 12 years if the storage temperature is 90 oC. HTF and VEKS therefore had either to implement a double layer of HDPE (proposed by the supplier) or find a new membrane material. The result was to use a recently developed Polypropylene (PP) membrane. This membrane was developed in the Austrian research project giga_TES and laboratory tests at JKU Linz showed a lifetime of more than 30 years at 90 oC.
The first version of the PP-membrane broke during winter 2020-21 because it was vulnerable to temperatures below zero. The total membrane therefore had to be changed to a version with additives and protected against minus degrees during water filling by spreading warm charging water from the edges. See Fig. 3
Fig 3. Protection of PP-membrane with warm charging water
In storages cooled down in winter the water surface will drop because of the temperature drop. This drop is smaller in Høje Taastrup. The top diffusor has therefore been changed so that the bottom of the lid is the upper part of the diffusor. This solution was laboratory tested and described in Urbaneck et al. Radiale Diffusoren in Warmwasserspeichern. Teil 1-3. Heiztechnik 2016. The diffusers also had to be placed at the end of the pit storage because of the narrow area as illustrated in Fig. 1.
The lid construction is similar to the refurbished lids in Marstal and Dronninglund, but tests at JKU Linz showed lifetime could be less than 20 years for the insulation material Nomalén. The supplier developed a new version with additives, that seems to prolong the lifetime with app. 70% according to tests at JKU Linz.
Last but not least a new business model was developed. The PTES is owned by VEKS and HTF (50%/50%). VEKS has bought the right to HTF´s share of the storage capacity for 20 years by paying an annual compensation to HTF. The partners have made a cooperation agreement in which it is stated that HTF takes care of the daily maintenance and monitoring of the PTES
All partners with an economic benefit from the storage pay a fixed annual amount for this availability for 20 years according to their share of the economic benefit. Transmission companies (VEKS and CTR/HOFOR) hold 56% of the benefit due to saved peak load. Four CHP producers hold 28% and three waste-to-energy plants holds 16% of the benefit. VEKS has made agreements with all parties.
The daily operation is optimized by Varmelast (a common owned company operating for the daily load dispatching in Copenhagen). Instead of daily forecasts now weekly forecasts for heat requirements, operational production capacity, power prices, fuel prices etc. are needed to optimize the system with the PTES.
