The thermal capacity of the physical heat reservoirs is based on thermodynamic properties of the storage material: specific heat and/or the latent heat.

1.2.1. Sensible heat storage.

While most of common materials, used for heat storage, have specific heat cp = 0,8 — 1,6 kJ/kg/°C, (0,23-0,44 Wh/kg/°C), cp of water substantially differs with cp = 4,19 kJ/kg/°C (1,16 Wh/kg/°C). Heat content of any reservoir utilizing sensible heat can be calculated as follows:

AHs = cp*At*m (2)

where m = mass of the storage volume and At = temperature difference between loaded and “empty” heat storage space. Thus when the sensible heat only is used for energy storage, 1m3 water 90°C can theoretically release max.104,7 kWh of useful energy after cooling it to 0°C.

1.2.2. Latent heat storage.

Latent heat of materials (phase transition heat) is usually much higher than specific heat. Thus the condensation of water vapour releases 2520 kJ/kg (700 Wh/kg) at10°C and freezing of water releases 334 kJ/kg (93 Wh/kg). Phase change materials (PCM) as energy storage medium are richly represented in search databases. European patent database Esp@cenet [3] returns more than
6000 citations of the term “phase change material” and Internet database Google returns over 7 millions citations. Systematic treatment of PCM for heat storage can be found in [4,5].

Water as PCM is used scarcely in spite of the large heat content and unlimited access. The reason may be the volume changes during the phase transition and low temperature of the fusion heat. Water expands about 2,15% (linearly) when converting to ice and about 1330 times at 10°C after evaporation at atmospheric pressure. The huge volume change makes it inpractical to use liquid to gas transition of water as heat storage medium in closed systems (1 kg water vapour stored at 100°C in 10 l requires a container dimensioned for the pressure at least 200 bar), but this phase transition occurs daily in the atmosphere. Water vapour content in the saturated air is about 0,4% at 0°C, 0,8% at 10°C and 1,5% at 20°C (w/w) or 1 kg H2O in approx. 130 m3 of the humid air.

The average temperature measured in Stockholm area year 2007 (59°27’N, 17°45’E) was 8,2°C and the average relative humidity was 82,2%. Average retrievable energy in each 100 m3 air is thus at least 2800 kJ (786 Wh), if the heat retrieval system can be cooled to temperature below 0°C. The sensible heat of the air contributes then with 36%, condensation of the water vapour with 56% and the fusion heat of the condensed water with 8 % of the total energy content. While the fusion heat of water does not contribute substantially to the heat retrieval from the moist air, the situation becomes very different, when the heat is retrieved from the soil. 1 m3 soil, saturated to 50% with the water, 15°C warm, can release 60 kWh when cooled down to -0°C. A small pit, 5x10x1 m, filled with humid soil, can thus supply enough heat for a single house during 1-3 winter months, if the water in the soil is allowed to freeze.

A domestic heating system, covering energy needs of a family house under a whole year, based on usage of solar energy, should utilize both direct insolation (mainly for summer hot water need and loading of the seasonal heat magazine), heat saved in humid air (during nights, autumn and spring months) and heat stored in the ground magazine during the winter months. Such a system has to contain heat pump and open solar heat collector connected in such a way, that the cold, expanded fluid in the heat pump circuit can cool down the heat absorbing surface of the SHC. The solar heat collector obtains thus double function: it collects the insolation and in the absence of the solar radiation it collects the heat from the air.