By Alfred Heller, Managing Consultant, NIRAS
The need for stabilizing the electrical smart grid increases the demand for very large thermal storages. Pit water storage are currently the most promising technologies according to IEA reports. The current article describes the adjustment of the seasonal thermal pits to meet the requirements from the future smart energy system.
At Dronninglund, Denmark, the PTES is placed into an old gravel production site that was repurposed for the current thermal water storage. With the truncated squared pyramid stub with a side length of 100 meters depth of 16 m, the volume is estimated to 62,000 m3. The pit is servicing the 1,350 housing units for large part of the heating season. The cost for construction was as low as 35 €/m3 for the pit, plus cost for connection and potentially additional buildings.
STATE OF ART FOR SEASONAL PIT THERMAL ENERGY STORAGE
There is a variety of Thermal Energy Storage (TES). Tanks (TTES), probably the most known technology, are very expensive and limited in size, both built as steel tanks and buried concrete tanks. UTES is storage utilizing underground structures and old mines for storing heat in water, and ATES stores heat and cooling in natural ground water aquifers. The pit thermal energy storage (PTES) is the most promising technology due to simplicity, height throughput, long-term storing abilities and, most importantly, low cost. Danish examples demonstrate the state-of-art for the technology in a small variety of detailed designs. Simplified, the idea is to dig a large cavity into the ground, line it to keep water inside the pit and put an insulation layer on the top to avoid heat losses. No insulation is made on the sides and the bottom of the pit in current designs. Due to the fact that the insulation materials cannot “survive” in contact with hot water, it has to be covered by liners, typically the same liner as used for the pit. At the surface of the whole lid construction, another liner protects the insulation materials against weather conditions, especially rainwater, snow and dust. Differences in design are applied with respect to lining and insulation materials and how moisture and rainwater is handled. Details on the design of pits and experiences with their applications are well-published in scientific publications, hence not repeated here. The focus in the current article is how to get the technology improved to meet the demands of the future sustainable energy systems.
Figure 1: Filling the pit with water at Dronninglund, Denmark, July 2015.
The application of the storage is a purely seasonal utilization cycle, where solar heat is produced in summer but delivered during a large part of the heating season. The feasibility of the technology is based on the assumption of a lifetime for the pits of 20 years. Hence, the specification for solutions was driven by these expectations.
In Figure 2, we find the temperature level that is typical for solar thermal pits.
All existing pits were designed for the purpose of solar heat production for low temperature district heating (DH). Such central solar plants (CSHP) produce heat by solar collectors placed in rows that ensure high possible outlet temperatures which a single collector would not be able to meet. However, collectors show high performance at lower temperature levels, but loose efficiency with increasing temperature levels delivered by the solar plant. Hence, the heat production is a compromise of efficiency for the solar collector field, the storage efficiency and capacity plus the temperature range for the district heating network.
Figure 2: (left) Temperature development over the first two seasons, 2013 and 2014 in PTES in Marstal, Denmark. Source: Planenergi 2019. (centre) Modelled top temperature of pit for one single seasonal period. (right) Many heating cycles during the year – abstract presentation.
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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