Should heat and cold be stored?

The storage of heat is commonly practised on a daily basis in “boilers”. However, long-term heat storage for the duration of a season (heat produced in summer and used in winter) is still too expensive in Switzerland. It is also possible to store cold for later use. This is common practice in industry on a daily basis for economic reasons, but is often not advantageous from an energy point of view. Seasonal cold storage is only possible for buildings (or remote heating networks) with a ground-source heat pump.

Our heating needs for homes are relatively constant during the day. On the other hand, they vary greatly throughout the year, depending on the season, with significantly higher needs in winter than in summer. For hot water, the situation is reversed. Our needs remain relatively constant over the seasons (we shower almost as much in winter as in summer), but vary greatly during the day. We use a large amount of hot water within a few minutes, just enough time to shower or run a bath. If we had to produce all this hot water at the time of use, we would need very powerful equipment to heat it up very quickly, which would be extremely expensive. It is much more advantageous to produce hot water in advance, with low-powered equipment, and store it in well-insulated boilers so that the stored heat is dissipated as little as possible. Daily storage of hot water is therefore imperative.

The same cannot be said for the daily storage of cold water. It is possible to produce ice at night and store it until noon the next day to cool the premises, replacing a conventional air-conditioning system. Similarly, refrigeration centres seek to cool below the required limit at night and turn off their refrigerators and freezers during certain periods of the day. These solutions are primarily of economic interest since they use electricity at night, when rates are low, and avoid using electricity at noon when rates are high. However, they are rarely relevant from an energy point of view, as they will generally consume more electricity in total.

What about the seasonal storage of heat and cold?

Several industrial processes produce year-round heat (incineration of household waste or sewage sludge), a significant part of which cannot be recovered in summer, because it exceeds our low needs at that time of year. This excess heat is therefore simply discharged, purely and simply wasted. In addition, we benefit in summer from a solar input that greatly exceeds our needs and, therefore, is not used either. From an energy point of view, it would be interesting to store this excess summer heat to cover part (or all) of our heating needs during the following winter.

Seasonal storage technologies aim either to heat the subsoil using geothermal probes or coils, or to store hot water in large, well-insulated tanks. This heat will be recovered in the following winter, either directly (in the case of the tank) or through the use of heat pumps (in the case of basement heating) [→ Q59].

Seasonal storage remains difficult to pay for, as it requires significant investment for a single charge and discharge cycle per year (hot/cold season). This makes the kWh of heat produced very expensive compared to daily storage, which runs hundreds of cycles per year for a much lower investment. As a result, seasonal heat storage is generally not yet profitable in Switzerland, although more and more projects are being developed. On the other hand, seasonal storage is already profitable in some countries (Denmark, Sweden, Germany) which have large heat requirements in winter and which benefit from low-temperature (50-70°C) remote heating networks. The high temperatures in most of the remote heating networks in Switzerland, often maintained for purely conservative reasons, represent a major obstacle to the use not only of thermal storage, but also of geothermal energy, environmental heat and various types of thermal waste.

Seasonal storage for cold is also possible. Storing the cold in winter to cool homes in summer is an easy and economical solution, provided that the building has a heat pump with geothermal probes [→ Q59]. The principle is simple: in winter, the heat pump extracts heat from the ground to heat the building, which in turn cools the ground. Simply circulating the water from the probes (between 2 and 5°C) in the summer is enough to recover the cold produced during the winter period and provide summer cooling while partially regenerating the heat from the ground.