Seasonal storage is of course possible with sorption and even more with chemical storage (see figure 1). But there is no solution yet mainly due to very little research in the field. Since 30 years seasonal storage has been investigated to reach high solar fractions, up to 100% solar, the final dream.

How big should a tank be to reach 100% solar? Many studies and experience have been conducted to solve this question. There are a few examples that actually are in operation! In the 80s it was calculated that a seasonal water storage should be as big as half the house, limitating its interest. That was indeed true but what happened is the stong demand reduction for heating a house. Savings of energy prior to investing for a new production device!

For a recent passive house, the volume is much less. With 20 m3 of water it is possible to reach 100% solar in mid Europe, in average meteorological year! Since climate variations can be as high as 25%, an auxiliary heating system is still recommended. It is interesting to use wood as back up if the user is interested to reach 100% by renewable energy!

When trying to deliver as much as possible heat from solar to more than one house, say a group of 100 houses, there are several technologies that have been investigated and a few pilot plants built around the world. They can be ordered in three generic families:

o the water based

o the soil or earth based

o the aquifer based (basically a mixture of water and soil!).

Accessing huge volume of water requires a tank! the biggest tank we can build are in the order of 20 to 30’000 m3 at a cost that is above 150 euros per m3. Which is not high considering a 1 m3 storage tank for a combisystem can cost 1000 to 2000 euros! But 30’000 m3 is not enough in certain cases like a solar district heating system say for 500 dwellings in Denmark or Sweden. Soil has adequate thermal properties at a low cost!

We have seen that heat capacity of soil is in the order of 60 to 80% that of water and thermal conductivity is comparable to that of water:

Soil can store sensible heat like water but is a bad insulation material. An underground storage system must therefore be completely insulated by some insulating material to withstand 6 months of duty must be very big! Minimum volume not to insulate bottom and sides of an underground storage is 20’000 m3 as an order of magnitude. This means a minimum of 500 dwellings connected to the store! The true parameter is the time constant of a heat store. For a store to be really seasonal, its time constant should be over 365 days! The next table shows all seasonal storage technologies that have been worked during the past 30 years..

Large water tanks have been built with success for solar plants in Sweden, Denmark, Germany and Switzerland. The specific cost of a large tank is much reduced compared to that of a small storage tank for a one family house, but still with 150 to 250 euros/m3 for 10’000 m3 tanks it is still expensive.

Duct storage are a cheaper and reliable solution. The german project Neckarsulm is proving well that the technology of a shallow duct storage is operating as predicted by computer models at a cost divided by 4 to 5 compared to a tank storage.

The Neckarsulm storage has 528 boreholes 30 m deep, and a top insulation only thanks to its huge size of 140’000 m3 in the long run. The present volume of store is 63’200 m3 heated up by 6’337 m2 of collectors. The core of the store reached 62C in summer 2003. The store operates with great heat losses during its warming up time (5 years) but is expected to reach a 50% efficiency in 2005.

Duct storage used with a heat pump are common in several countries. The store operates at low temperature (0 to 30C), thus without any heat losses!

Aquifer storage was the most promising technology in the 80s because of its low cost to acces huge volumes of water.

China has a long tradition of cold water storage in aquifers. In Europe and the US, pilots plants to test the concept of storing solar heat without a heat pump at 60 to 90C into an aquifer at 30 to 200 m depth, were built in Denmark, France and Switzerland with limited success due to chemical problems (calcite precipitation) or due to buoyancy effects ruining the exergy of the store.

Recent projects in Germany prove to work (almost) well if design is carefully based on the results of the site investigations. But still a strong natural groundwater flow negatively influences the results, and the storage without a heat pump would probably not reach a reasonable efficiency. Cold storage in aquifers has become a cost effective technology in some countries. Competition against electricity costs makes the alternative of free cooling with a COP of more than 20 attractive

Rule of thumbs for large seasonal storage are:

Minimum load: 500 MWh recommended

1.5 to 2.5 m2 solar collectors per MWh of load for 40 to 60% of load

1 to 2 m3 per m2 of solar collector Thermal insulation: min. 40 cm at 0.04 W/mK

Solar productivity: 200 to 300 kWh/m2 in mid Europe climate Storage cost : 150 to 250 euros/m3

1.5 to 3 m2 solar collectors per MWh of load for 40 to 60% of load

2 to 6 m3 of storage per m2 of solar collectors minimum 20’000 m3 if insulation only on top

15 to 50 W/m of borehole (double U-pipe, quartz bentonite filling) Storage cost: 30 to 60 euros/ m of borehole

Aquifer storage 2 to 6 m3 of water per m2 of collectors

Minimum 50’000 m3 of storage for a no heat pump system

Minimum depth: 10 to 20 m

Should preferably be used for cold storage

Storage cost: 5 to 20 euros/ m3 but very very dependant on local conditions A strong Regional groundwater flow (1 mi/month) can ruin the store

Regarding large seasonal storage, the present conclusions after many pilot plants are the following:

o Water technologies are reliable but more expensive. Water tightness is the main problem that can be overcome with stainless steel liners or special very dense concrete

o Pit storage have some potentials in terms of cost /performance ratio but are difficult to master

o Duct storage are the most simple. With heat pumps they work fine at low temperature range (0- 30C). The challenge of a no heat pump solar system is proved to be possible in the Neckarsulm project and in the Drake Landing project in Canada.

Aquifer storage is the cheapest technique for huge volumes, when it can be mastered. For cold storage, cost effectiveness is proved (payback time less than 5 to 7 years in Germany, Sweden, Netherlands on more than 200 installations). For warm storage (40 to 90C), the chemical composition of the aquifer and its geometry (natural convection problem) are very determinant and local conditions prevail so that a prognosis is always difficult to make. On site investigations and modelling are required making the process long and somehow costly for a result difficult to assess a priori.

Solar ponds (salted water) are also candidate for long term storage in desert.

High temperature storage (concrete or molten salts) will also be needed for thermal power plants at 400 to 500C to overcome clouds or cloudy hours. There are also preferred chemical reactions (ZnO) investigated since decades but with no commercial solutions yet.

3. Conclusions

Sorption and chemical storage are future candidates for solar heat storage. However recent work from IEA SHC Task 32 has shown that new materials are urgently needed. There are also other ways to indirectly store solar energy, i. e.using hydrogen as a storage material and a vector of distribution.

Water is still the best storage material for small scale solar installations.

There are ways to improve water tank storage efficiencies but we are reaching limits for diurnal storage. The progress will be more in the overall system and control strategy. The heat exchange also is a research topic.

PCM materials will have a come back in construction elements for cooling and less for heating. For combitank the competition with water is hard to win due to a large operating temperature range.

There is a need for some new investigations in sorption materials, where silicagel and zeolites have shown too limited possibilities. Chemical stores are also back in the R&D work but unfortunately to a low level.

For large seasonal storage, duct storages have an interesting future with or without heat pumps, and aquifer storage are best suited for cold storage and free summer cooling.

Seasonal storage for one family house is possible through an insulated water tank, but economics show systems with short term storage that can cover up to 50% of the load as more attractive at present. In the long run, chemical storage will find its way to the one family house but there is more R&D budget to allocate to this fundamental topic for a large deployment of solar in mid european climates.

References

[1] Hadorn J.-C. editor, (June 2005), Thermal energy storage for solar and low energy buildings — State of the Art, a IEA SHC Task 32 book, Printed by Servei de Publicacions Universidad Lleida, Spain, 170 pages ISBN 84-8409-877-X, available through Internet www. iea-shc. org Task32

[2] IEA SHC Task 26 (2004): Solar Heating Systems for Houses — A Design Handbook for Solar Combisystems, W. Weiss and al., James & James, 2004, 313 pages

[3] Task 32 reports are available at http://www. iea-shc. org/task32/publications/index. html

[4] Solar Engineering of Thermal Processes, John A. Duffie, William A. Beckmann 1991, 2nd edition, Wiley — Interscience

[5] De Winter (Ed.) Solar Collectors, Energy Storage, and Materials June 1991, MIT Press.. Historical! A must…

[6] Solar thermal energy storage, HP Garg, SC Mullick, AL Bhargava, D. Reidel Publishing Company, 1985

[7] Streicher Wolfgang, Sonnenenergienutzung, lecture book, Graz University of Technology, 2003

[8] Central Solar Heating Plants with Seasonal Storage : Status Report by Jan-Olof Dalenback, International Energy Agency, Book — January 1990

[9] Guide to seasonal storage, JC Hadorn, 1988, published in English by Public Works, Canada

[10] Thermal Storage Of Solar Energy. C. Den Ouden. Holland, The Hague, Martinus Nijoff Publishers, 1980

[11] Drake Landing Project, Canada http://www. dlsc. ca/