HOT|COOL NO. 4/2019 - "Components"


TECHNOLOGY: STATE-OF-THE-ART Heat storages can be constructed in different ways – tanks, pit, borehole and aquifers as shown in fig 1.




Storage efficiency

91 %

90 %

No. of storage cycles



87 / 12 O C

89 / 12 O C

T-max. / T-min.

Heat capacity with T-max./T-min. 5,200 MWh

5,400 MWh

Fig.3: Dronninglund – PTES energy flow 2016 and 2014-20

INNOVATION IN THE FLEX_TES PROJECT The FLEX_TES project will demonstrate how the PTES, which was originally developed for longterm solar thermal storages, can be utilized to add flexibility to a large DH system with many different producers connected to the DH system. The PTES will be established in Høje Taastrup – a municipality in the western part of Greater Copenhagen - and integrated with the local DH distribution company Høje Taastrup DH Company as well as with the VEKS heat transmission system. The business case study for the establishment of the PTES has shown that the benefit of the PTES will be significantly higher if the operation of the PTES is integrated in the daily optimization of the large interconnected DH system in Copenhagen. The business case has shown that in order to obtain the highest benefit for the system the PTES will need to be operated to store/discharge energy on a weekly and daily basis. As part of the preparation of the project the operation of the PTES has been optimized using the software model Balmorel, which provides an economic optimization of the electricity and DH market. The model optimization shows that the PTES should be charged and discharged 25 times each year in order to have the highest benefit (fig. 4).

PTES has during the last 30 years been developed in Denmark as longterm storages for solar DH. The Technology Readiness Level (TRL) is 8 for PTES combined with solar thermal plants, but still there are problems to be solved. These are: • Lifetime for the PE-insulation in the floating lid. • Water ponds on top of the lid. • Oxygen in the storage water causing risk for corrosion in connected steel pipes.

The FLEX_TES project will bring PTES used as accumulation tanks from TRL 6/7 to TRL 8.

Full-scale PTES has been implemented in connection to: • Marstal DH Company in 2011-12 (75,000 m3 water) • Dronninglund DH Company in 2013-14 (60,000 m3 water) • Gram DH Company in 2014-15 (122,000 m3 water) • Vojens DH Company 2014-15 (220,000 m3 water) • Toftlund DH Company 2016-17 (70,000 m3 water). The results of the monitoring program in Dronninglund gives the energy balances for the three years 2014-2016 as shown in fig 2 and fig. 3:

Fig. 4: Modelled energy content in the storage during the year in Høje Taastrup with advanced oper-ating strategy.

This operating pattern for the storage is technically possible but it involves different challenges and needs to be developed and demonstrated. The PTES will work as an accumulation tank and will be charged and discharged several times during a year. That will change the water temperatures from being 85-90°C in a couple of months to constantly 90°C in the top of the storage. The system benefit of the PTES is via BALMOREL analyzed to be 6-6.5 mill. DKK/year (0.8 million €/year) using 2025 as an average reference year.

Fig.2: Dronninglund - PTES energy flow 2014 – 2016 (Source Solites)

All PTES’s in Denmark are constructed with soil balance and floating lid. The liner material for water tightening is a welded HDPE-liner, and the lid construction is either with PE- foam plates as insulation (Marstal and Dronninglund) or with expanded clay (Gram, Vojens and Toftlund).

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