Seasonal storage is a key component in the transformation of today’s energy industry. Besides storing energy in summer for heating in winter, it can also be used to save waste heat from the industry and to increase the electricity production from biomass CHP plants. Experiences gathered with the technology during case studies were summarised as part of the study Seasonal thermal energy storage – Report on state of the art and necessary further R+D, which was published by Task 45, Large Scale Solar Heating and Cooling Systems, of the IEA SHC programme. Together with the Guidelines for Materials & Construction on the two most common storage types, borehole (see the chart) and water pit, it provides a good overview of the current advancements in this field. Additional research into the design of seasonal storage will be carried out in follow-up Task 55, Towards the Integration of Large SHC Systems into District Heating and Cooling (DHC) Network. Interested stakeholders have been invited to join the kick-off meeting of Task 55 in Graz, Austria, between 19 and 21 October (see contact details below).
Researchers from German institute solites analysed seven case studies of seasonal heat storage set up in Canada, Denmark and Germany and have summed up the requirements for further R&D as follows:
- Develop materials for storage construction and lining which can withstand 100 °C for 30 years
- Design inexpensive construction technologies
- Further develop charging and discharging
- Expand TRYNSYS models
Borehole storage: Soil type and shape have moderate impact on overall performance
The solites report on current technological advancements is a good introduction to the topic and worth reading before delving into the technical details of the guidelines. The 15-page guidance on boreholes identifies eight large solar heating systems – three in Germany and one each in Canada, Denmark, Finland, the Netherlands and Sweden. It is published by Bruce Sibbitt and Doug McClenahan from Natural Resources Canada.
Borehole storage uses the heat capacity of large amounts of local soil to store thermal energy underground, since the soil comes at no cost and is available on site. What needs to be invested in is the drilling of the boreholes for the heat exchanger to transfer heat to and from the soil and into the insulation, which is placed on top of the storage. Existing borehole storage areas of 50,000 m3 have cost between 20 and 35 USD/m3 – figures that can go down to between 10 and 15 USD/m3 for ground storage above 250,000 m3, as simulations show.
More than 200 in-depth TRNSYS simulations have pointed to the soil type as well as the shape of the borehole storage having moderate impact on overall system performance when it comes to storage areas with more than 70 boreholes on a plot. One example from the report: “A light, dry soil can provide high storage efficiency by having relatively small losses. However, the same soil properties significantly restrict the ability to accept heat input and to deliver heat as required, while forcing collector operating temperatures to increase and collector efficiency to drop.”
Pit heat storage: Liner quality determines economic life
Pit heat storage is a hole in the ground covered by water-proof lining and filled with water, which in turn is covered by floating insulation. To minimise the cost of soil handling and transportation, the excavated soil from the bottom of the pit is used to create embankments around the upper part of the storage area. The most expensive part of the storage is the cover, which may consist of either flexible or rigid insulation components or bulk insulation. The cost of pit heat storage areas depends very much on the size of the tank: Whereas a 75,000 m3 tank is 36 EUR/m3, the figure drops to 28 for 200,000 m3.
The critical aspect to providing a trouble-free and durable storage solution is the lining, Morten Vang Jensen from PlanEnergi and author of the 31-page study Seasonal Pit Heat Storages – Guidelines for Material & Construction emphasise: “Polymer and elastomer liners are by far the cheapest regarding both material price and installation cost, but metal liners have an advantage regarding long-term stability and vapour tightness.” The Danish Technological Institute has developed a method for accelerating tests of the service life of polymer liners for pit heat storage. The aforementioned study, which was published in December 2014, says: “The best liners tested by this method so far are specific high-temperature developed HDPE liners, which have an expected lifetime of not more than 3 years at a constant temperature of 90°C.” This is said to correspond to “a lifetime of 20 years” based on a typical temperature profile of pit heat storage areas connected to a solar plant.
In addition to the lining, the authors of the guideline also list other requirements which will have to be met during planning and construction of pit heat storage:
- Ensure floating ground water above bottom level of storage to reduce losses
- Soften water treatment during filling to avoid rust on steel parts
- Carry out a soil compaction test during construction to ensure stability of embankments and to determine inclination
- Perform simulations using TRNSYS and type 342 (multi-flow stratified thermal storage) to predict performance of pit heat storage
More information at:
http://task45.iea-shc.org
http://task55.iea-shc.org
TASK 55 Kickoff meeting: Contact operating agent Sabine Putz from S.O.L.I.D for more information about the kick-off meeting in Graz, Austria, between 19 and 21 October: s.putz@solid.at