Seasonal Thermal Storage

In previous posts, we’ve mentioned our active interest in alternative heating and cooling methods, including projects by SANAA and Peter Zumthor incorporating thermally activated facades. Our post on Werner Sobek’s HausR128 briefly discussed Seasonal Thermal Energy Storage, a system of collecting excess heat or cold generated in one season, to heat or cool in another. In Sobek’s house, copper piping along the ceilings absorbs excess heat during summer and transfers it to a highly insulated storage tank. In the winter, this collected heat is recirculated to warm the house. While the idea isn’t new, as a substitute to burning fossil fuels, we find it highly intriguing.  We’ve noticed an uptick in the number of projects incorporating seasonal thermal storage, which seems fairly successful at reducing or even eliminating traditional mechanical systems.

Seasonal thermal storage differs from diurnal storage, which offsets heating/cooling loads within a 24 hour period (e.g. trombe walls). Seasonal storage requires significantly larger storage mediums and temperature differentials. An effective way to offset the cost of seasonal storage is through district heating. The development of phase-change materials (PCMs) should decrease the volume needed for storage. The International Energy Agency’s Task Force 32, Subtask C researched the feasibility of different PCMs through 2008, a number of their reports can be downloaded here.    


One of the first modern uses of seasonal thermal storage was MIT’s Solar House #1, built in 1940. This small project included a large tank beneath the floor which was used to heat the building year-round.  Solar collectors on the roof were used to heat the water in the tank. Not overly elegant, but innovative and worth noting.


From 1988-1996, the International Energy Agency ran a series of demonstration projects (Task Force 13) to monitor seasonal storage concepts. Germany’s Nullheizenergiehaus (zero heating energy house) in Berlin-Spandau, utilized a 20,000l insulated storage tank, which was charged by roof-integrated solar collectors. This article (PDF, German) includes photos and diagrams that may be of interest, however results seem to have been compromised due to occupant error.

Germany ran two additional programs to further study and monitor seasonal storage utilizing Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES), Tank Thermal Energy Storage (TTES) and Pit Thermal Energy Storage (PTES). These programs, Solarthermie2000 and Solarthermie2000Plus, provided much-needed research into constructability, cost-effectiveness and efficiency of these systems. 

Aquifer Thermal Energy Storage (ATES)

An ATES system uses a heat pump and drilled wells to displace heat or cold to contained, subterranean aquifers. ATES operates as a  ‘closed loop’ installation, where water that is withdrawn is returned to the same aquifer. ATES systems can be very cost-effective, with typical payback periods shorter than 10 years. They have been utilized successfully on a number of projects including several hundred installations in the Netherlands and Sweden.  

Norman Foster’s renovation of the Reichstag utilizes both warm and cold ATES systems, which are large enough to supply adjacent government buildings as well. Excess heat generated by the building is stored in an aquifer, where it is used to warm the building in winter. Cold water from an absorption cooling plant can be stored in a separate aquifer to run ‘chilled ceilings’ in summer. Geothermie Neubrandenburg GmbH has a fairly technical report available describing the system layout and efficiency (PDF).   

The Danish Broadcasting Corporation’s Media House has a year-round cooling demand, which is met by a hybrid system of night flushing and a cooling-only ATES. Completed in 2006, the ATES is expected to have a payback of 12 years while removing over 1,500 tons of CO2 per year.

3xN’s Muziekgebouw in Amsterdam is part of a larger mixed-use complex that is cooled and heated by ATES. Photos and diagrams of the system can be found here (PDF, Dutch)

Borehole Thermal Energy Storage (BTES)

Borehole Thermal Energy Storage (BTES) is a thermal storage system that utilizes an array of deeply drilled holes. These holes are filled with one or more sets of closed-loop tubes to transfer the heat, and are then filled with a grout or similar medium. If used for heating, the BTES is charged in warmer months and stored in the ground til winter. If used for cooling, heat is extracted during winter, and added in summer by ‘free cooling’.    

Outside of Calgary, the community of Drake Landing has incorporated a BTES system of 144 boreholes that will supply 90-100% of the space heating needs, as well as up to 70% domestic hot water needs (DHW). Solar collectors on the garages (why?) will charge the system during summer months. While we find the design of the houses rather lacking, it is great to see developers building this system in the heart of Canadian oil.    

Tank Thermal Energy Storage (TTES)

TTES is a system utilizing highly insulated concrete, GFRP or steel tanks to store heat/cooling until needed. In the photo above, a TTES in Hannover (DE) incorporates public play areas. Sobek’s systems is a TTES, as is this passivhaus project underway in Galway (IE). This ‘super’ passive house is planned so that nearly all space heating and DHW will be supplied by solar energy. The Galway project’s 23m³ (760 ft³) underground tank is concrete with 400mm-600mm of insulation around the exterior achieving an average U-value=0.059W/m²K  (R-97!).

These strategies work well in concert with other low-energy practices, including super-insulating and airtightness. The environmental, economic and health advantages of these systems appears to be highly favorable compared to traditional mechanical systems. With a reduction in electrical demand for space heating, fewer harmful gasses and CO2 are emitted. Economic payback can be as low as 3 years, and with long life expectancies, can deliver significant savings for several decades. Reduction or, better yet, elimination of traditional ducted mechanical systems makes for healthier buildings. Outside of all this, from an architectural standpoint, the removal of hideous cooling towers and roof top mechanical units is a definite benefit. Additionally, pits, tanks and boreholes can be hidden under parking lots, parks or buildings – truly an energy-efficient minimalism we can appreciate!    

It was recently brought to our attention that here in Seattle, the Roosevelt Reservoir will be decommissioned in the next few years. We feel this could become an excellent solar thermal storage demonstration for the Northwest. The reservoir could be filled, insulated and covered with a public park for the neighborhood.  Nearby houses would run piping from rooftop solar collectors to the reservoir/tank, storing the intense solar energy from summer until our cold, gray winters. For us, it is this kind of thinking – this ‘double duty’ – that makes seasonal storage über-sexy.    

Further Reading:
  • Proposed application for South Kensington’s museum district (BD Online)
  • Institut Für Thermodynamik und Wärmetechnik report on Germany’s R&D programs (PDF)
  • Solar und Wärmetechnik Stuttgart (SWT) report on Solarthermie 2000 (PDF)
  • IEA’s Task 13 Final Task Management Report (PDF)
  • Report on the rehabilitation of Malmo’s Bo01 district, which is supplied by 100% renewables including an ATES (PDF)
  • EU SAVE Programme & Nordic Energy Research: Pre-Design Guide for Ground Source Cooling Systems w/ Thermal Energy Storage

One response to “Seasonal Thermal Storage

  1. namhenderson

    You guys should seriously propose this (the reuse of the reservoir) to city or other oversight parties…