Interseasonal solar storage

last updated: 10/98


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Solar energy is cleanest and most environmentally friendly power source available. Recent advances in the design of photovoltaic cells and vacuum-type heat pipes, and the efficiency of passive solar houses have meant that solar energy is now coming of age and can play a useful role in the design of new, more energy efficient (or self-sufficient) buildings. This is particularly true in low latitudes and tropical areas, where heating requirements are low and sunlight constant for most of the year. However, in Japan and Europe there are still major challenges to be overcome in the design of solar buildings. The principal problem is that the weak winter sun and relatively short daylight hours mean that there is simply not sufficient sunlight to meet buildings’ energy demands. Thus, to develop a self-sufficient solar house in Japan or in Europe requires use of both ‘passive’ and ‘inter-seasonal’ technologies. Passive design aims to reduce the energy demand of the building through advanced insulation, while ‘inter-seasonal’ technology attempts to store solar energy from summer months for use in winter heating. The successful blend of these two technologies is an exciting and important step on the road to the future development of truly sustainable communities.

Passive Design I - concepts

In any attempt to use solar power for providing energy at high latitudes, it is important for the design of the buildings to be as energy efficient as economically possible. It is now widely recognized that there is enormous potential for reducing the energy consumption in both residential and commercial buildings. Although 250 kWh per m2/year is used to heat an average German (north European) apartment, the demand drops to 50 - 100 kWh per m2/year in buildings conforming to the new thermal insulation regulations. However, new purpose-built passive houses can achieve much higher levels of efficiency/ . For example, the ‘passive house’ in Darmstadt- Kranichstein (a four-apartment house with very good thermal insulation and ventilation heat recovery) requires only 12 - 15 kWh heating energy per m2/year.

Passive Design II - The self-sufficient solar house

For more than six years, the Fraunhofer Institute for Solar Energy Systems (based in Freiburg, Germany) has been involved in building and testing of a ‘Self-Sufficient Solar House’ (see Figure 1). The project aimed to show that the total energy required by a single family house could be provided by solar sources and was funded by the German Federal Ministry for Research and Technology. The starting point for the project was construction of an extremely energy-efficient “passive house”.

figure 1

The two-storey building has a living area of 145 m2 and was built with very good thermal insulation, which gave it a high thermal inertia (making it slow to react to external temperature changes). The walls were constructed from lime-sandstone bricks (efficient heat storage) and the ceiling of the building was insulated with glass foam. The northern side of the building has additional cellulose insulation, installed in a ventilated gap behind the exterior wooden cladding.

The building was designed to be very airtight and any air entering or leaving did so via an underground heat exchanger which recovers 85% of heat from any air leaving the building and uses it to pre-heat incoming air. The overall shape was squat and compact, and the house faced the south to collect maximum possible solar energy. Almost all of the south wall is coated with transparent insulation (TI). By comparison the north wall has only a few triple-glazed windows. The TI allows the wall to act as a heat collector during the day and a large radiator during the evening. Because the area of TI is so large, it means that the air in the building feels warm, even if it is only 18 - 19°C. During the summer, blinds for the TI and windows prevent the house from becoming uncomfortably warm.

Altogether these measures reduced energy consumption of the ‘Self-Sufficient Solar House’ to less than 5% of that required for a conventional German apartment (12-15 kWh/m2 compared to the usual 250 kWh/m2). However, all materials used were commercially available — so it is clear that such levels of performance could easily be equalled by other buildings.

Interseasonal storage I - concepts and materials

The storage of solar energy over the long-term has always been problematic. For short-term (ie day-night) storage, batteries have proved extremely useful. However, such batteries are simply not suitable for storing the large amounts of energy required for heating a building over the winter months. The simplest solution is to store solar energy as either ‘sensible’ or ‘latent’ heat.

‘Sensible heat’ increases the temperature of a solid or a liquid (e.g water). In concept it is similar to the ideas used in passive building — but the heat must be retained over a much longer period than that required for thermal walls or slate floors. Therefore the material heated needs to be stored in an extremely well-insulated container. The following substances are widely used to store sensible heat:

Table 1 - Materials for storing Sensible Heat

Material Specific Heat Capacity (kJ/kg) volume (m3) to store 106kJ
oil 1.1-2.1 32-64
stone/rock 1.0 30
water 4.2 16

‘Latent heat’ is the heat associated with the phase change of a material (such as ice changing into water). The advantages of storing energy as latent heat in phase change materials (PCM) is that a great deal more energy can be stored. For example, it requires 80x more energy to convert 1kg of ice into water than it does to raise the temperature of water by 1 degree Celsius. Salt Hydrates and Paraffin are two materials which are attracting much interest for domestic space heating because of their relatively low cost and the suitable temperatures (20-60°C) at which they undergo the transition between liquid and solid. However, use of PCMs for inter-seasonal heat storage is not yet widespread — it presents many more technical problems (such as supercooling and phase segregation) than does sensible heat storage. For this reason, most existing systems use sensible heat storage — latent systems will not be further discussed in this article.

Interseasonal Storage II - Early experiments

The Self-Sufficient Solar House in Freiburg has already been discussed. However, even with this efficient passive design the need for additional heating of the house in winter was recognized. Thus in a mid 1990s overhaul of the house the decision was made to install a revolutionary system for long-term storage of solar energy.

The scientists and technicians working on the house soon developed a hydrogen/ oxygen energy storage system. Electricity, generated from standard photovoltaic cells, is used to decompose water into hydrogen (H2) and oxygen (O2). These gases are then stored in separate tanks to provide fuel for space and water heating. In the winter, flameless combustion of the hydrogen is used to heat the inlet air in the ventilation system with a catalytic hydrogen diffusion burner (1.5 kW). If additional electricity is required, it is gained from the reaction of hydrogen with oxygen in a fuel cell. As it is only operated during periods with little sunshine, the heat losses from the fuel cell of about 40 % (at a temperature of about 70 °C) can be used to help heat the hot water via a heat exchanger (heat/electricity cogeneration).

The Frieburg experiment has clearly showed that inter-seasonal storage of solar energy is possible. Even in the cool German climate, it is possible to generate all required energy from solar sources. In fact, since 1997, the house has been selling excess energy back to the utility companies. However, the technology was expensive, and the storage of large amounts of combustible gases in residential buildings raised questions of safety. The search was now on to find safer, cheaper ways of storing energy for the winter months.

Interseasonal storage III - storing sensible heat in water

In choosing a storage medium for collected solar energy, the use of water has many advantages over hydrogen and oxygen gas:

• it is unreactive and extremely safe
• it has an extremely low cost
• its high specific heat capacity means much energy is stored in a relatively small volume
• no toxic materials to deal with on future demolition work

As stated earlier, if water is to be used for long-term storage of solar energy, it must be stored in an extremely well-insulated container. In normal conditions, this means underground. To maximise solar gain on water heated, and to gain maximum benefit in using the water in radiators, care should be taken not to destroy the stratification that develops in the tank (whereby hot water moves towards the top of the tank and cold water towards the bottom).

Interseasonal storage in water I - office building case study

In 1998, the first results came in from a team of Italian researchers from the engineering Department of the University of Calabria, for the use of inter-seasonal storage in an office building in Northern Italy. Key statistics (shown in Figure 2) of the system are:

Solar collectors: 91.2 m2 vacuum-type heat pipes
Storage Tank: 500 m3 reinforced concrete with 0.2m foam glass insulation
Building Volume: 1750 m3
Heating Requirements: 111GJ

figure 3

During the months between April and October the space heating in the building is shut off, and all collected solar energy is used to increase the temperature of the water in the underground storage tank. Assuming a starting temperature of 30°C, the water will be heated to more than 80°C by the end of October. During the winter months solar energy is still collected, but is insufficient to meet the heating demands of the building. Hot water is drawn from the top of the tank to feed radiators in the building - after the heat is extracted it is returned to the bottom of the tank. This thermal stratification is much more energy-efficient and means that the hottest water is always available for building heating, and the coldest water is always that drawn up into the solar panels (allowing maximum heat gain).

Figure 3 (below) shows the energetics for the house over the first year of operation. Points to note include the low temperature of the storage tank at the start of the experiment (11.6°C), which resulted in a maximum tank temperature for the summer of 71.6°. However, at the start of year 2 of operation, the temperature in the tank was significantly higher (more than 30°C) suggesting peak temperatures in excess of 80°C will be achieved. Another point is the loss of collected energy, even with high levels of tank insulation. Over the first year total losses were 113GJ of the total 257GJ collected at the solar panels.

figure 2

Nevertheless the total efficiency of the system is 25%, comparable to the values obtained for thermal power stations. The experiment has clearly shown that interseasonal storage of solar energy as sensible heat is a practical method for creating energy self-sufficient buildings.

Interseasonal storage in water II - district heating

In Northern Europe, particularly in Sweden and Denmark, there is a long tradition of district heating in the winter (where a central unit provides heat to a number of buildings). Most such systems rely on the pumping of warm water to buildings and are suitable for replacement of the central boilers with ‘interseasonal’ solar storage systems.

An advantage of planning inter seasonal systems for large residential communities is that it is possible to use larger (and much more efficient) collector than is possible on the small roof space provided by individual family homes. It is also possible, with multi-unit residences, to construct buildings which have a lower surface area to volume ration and therefore a higher thermal inertia.

There are already a number of solar district heating schemes in operation in Europe, which are discussed below (see also Figure 4). The older projects (built in the 1980s) clearly use less-efficient materials than the newer projects and consequently provide lesser proportions of total energy demand. They are included however to show improvements in systems and technology.

Table 2 - Large-Scale Community Solar Heating Projects in Europe

Location Built Number of buildings CollectorArea(m2)/
Storage Area (m3)
Hammarkullen, Sweden 1986 400 apartments 1500 / 80 40% DHW
Ravensburg, Germany 1992 136 apartments 257 / ? 35-45% DHW
Saro, Sweden 1989 48 apartments 740 / 640 35% TOT
Neckarsarlum, Germany 1993/4 325 apartments 700 / ? 50% DHW
Hamburg, Germany 1995-7 120 houses 3200 / 4500 64% TOT
Friedrichshafen, Germany 1995-7 570 apartments 5600 / 12000 48% TOT

DHW - Domestic Hot Water
TOT - Total Energy (Heating + Hot Water)

In current schemes is estimated that such systems could provide of 50-80% of total energy needs for heating and domestic water-heating (for washing, kitchens etc.). The theoretical requirements for such a system are listed below

•installation of 0.2-0.3m2 of solar collectors per m2 of heated building
•initial investment of 150-200 ECU/m2 (construction of collectors etc.)
•storage tank volume of 2 m3/m2 of heated building (cost depends on tank material)

Interseasonal storage in water III - district heating case study

In Spring 1995 construction started on the new ‘Bramfeld’ housing project in Hamburg, Germany (see Figure 5). Plans were drawn up for 120 houses of modern energy-efficient design (heating energy requirements 42kWh/m2/year) receive more than 60% of total energy requirements from solar sources. A total collector area of 3200m2 was installed (cost DM 4.0 million) and hot water produced was stored in a tank of 4500m3 (cost DM2.1 million). Assuming a 20 year life for the system, it delivers electricity at a cost of US$0.25kWh. Although this unit cost is quite expensive, it must be remembered that the energy requirements for these houses are less than 20% of those for typical German apartments, so total cost to the owner will be lower than for conventional heating of a conventional apartment.

figure 4

Predictions of the system suggest that water in the central storage will reach 95°C. Water supplied to the radiators will be at a temperature of at least 60°C and water will return to the bottom of the storage tank at 30°C. The storage tank used in the system is shown in Figure 6 (below). The use of a stainless steel liner means that the tank is vapor tight (in contrast to prototype systems which used plastic liners). This means allows that water-resistant insulation is not necessary, and that relatively cheap insulation materials (such as polystyrene) can be used. Obviously however, areas in direct contact with the liner require use of mineral wool, as the temperatures are too high for polystyrene.

figure 5

People and companies

A comprehensive list of European projects in operation (and the companies involved) can be found at


The development of inter-seasonal storage of solar energy in Europe is an important step in the development of sustainable communities and energy self-sufficient buildings. The projects mentioned in this report are mostly experimental systems, funded by Government Departments. However, the results of these pilot projects show that the technology works and is commercially viable. This, combined with the commitment of European governments to renewable energy suggests that such designs will become a major growth area in European construction over the next 10-15 years.

Although revolutionary in reducing the energy requirements of multi-family dwellings the pilot projects have showed that such advances do not require vast amounts of research and development spending, or development work in new materials. Thus, this sort of project is likely to be particularly attractive to construction companies throughout Europe and other mid-latitude countries. This will be even more the case if governments offer incentives and assistance to such ‘green’ construction. We will keep you informed of the development of such ‘green’ construction in Europe in future issues of the Fujita Research Report.

References (The Self-Sufficient Solar House, Freiburg) (European Large-scale Solar Heating Network) (Hamburg Project)

The self-sufficient solar house in Freiburg - Results of 3 years of operation’ Solar Energy, 1996, Vol.58, No.1-3, Pp.17-23

‘High temperature water pit storage projects for the seasonal storage of solar energy’ Solar Energy, 1997, Vol. 61, No.2, pp 97-105

‘First experimental results from a prototype plant for the interseasonal storage of solar energy for the winter heating of buildings’, Solar Energy, 1998, Vol. 62 No.4 pp 281-290

‘Performance analyses of sensible heat storage for Thermal Applications’ International Journal of Energy Research, 1997, Vol. 21 pp 1157-1171

‘Review on sustainable thermal energy storage techniques: Part I Heat storage materials and techniques’ Energy Conversion and Management, 1998, Vol. 39, No. 11, pp 1127-1138

Godfrey Boyle (editor), Renewable Energy - Power for a Sustainable Future (1996)


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