Sensible Heat Storage for Low Temperatures
(up to 150 °C)
General Description
Mode of energy uptake and output: Heat-to-heat
Summary of storage process: During charging (energy supply) the sensible heat storage, the temperature of the storage content increases. Materials suitable for storage applications are non-toxic and inexpensive with a high specific heat capacity c in kJ/(kg∙K), for which even a small increase in temperature results in a large quantity of heat Q in kJ or Wh being stored. For water, with c20°C = 4.2 kJ/(kg∙K) per 1 kilogram of water and 1 degree Celsius temperature increase, Q is 4.2 kJ or 1.17 Wh. For a 500 litre water storage tank and a temperature increase of 70 K (e.g. from 20 to 90 °C), this amounts to 41 kWh of heat stored at 90 °C. Discharging (energy withdrawal) cools down the storage tank’s contents. For liquids, density changes depending on the temperature. Hot water is lighter than cold water. Thus, buoyancy forces cause thermal stratification in the storage tank. This natural stratification should not be disturbed during charging and discharging, as otherwise the mean temperature in the storage tank will drop to a lower level, at which it may not be usable without additional reheating.
System design: Generally, storage systems should be distinguished according to whether they hold heating water or domestic hot water. However, a combination of the two is equally possible, as illustrated in Figure 2 (source: ITW Stuttgart). For domestic hot water storages, charging and discharging takes place indirectly via an integrated heat exchanger. These spiral pipes, usually made of steel, heat the required domestic hot water through the heating circuit or solar thermal system. Since the hot water is stored directly, the tanks are manufactured from stainless or subsequently enamelled steel for hygienic reasons. Additionally, for legionella protection, the contents of the storage tank must be heated to at least 60 °C once per day.
Storage systems integrated into the heating system mostly employ direct charging and discharging. A typical application would be a buffer storage in a wood combustion system which is equipped with an additional heat exchanger for the solar thermal system, as shown in Figure 2 “Heating buffer”.
The storage tank may therefore be charged by several heat sources. The domestic hot water storage is then again supplied indirectly through a heat exchanger. Due to the physical separation between domestic hot water and heating water, the buffer tanks may be made of less expensive sheet steel. Newer systems usually replace the large domestic hot water tank with smaller systems such as a fresh water station, combined storage tanks (small domestic hot water tank integrated into the heating storage tank) or hygiene coils in the buffer storage tank.
Advantageously avoiding the Legionella problem, these systems provide domestic hot water on demand. Maintaining thermal stratification is, however, absolutely necessary with these systems in order to always keep enough energy in the storage. So-called stratified charging units ensure that the hot water is directed to the desired zone in the storage and that mixing of the thermal strata is reduced to a minimum thanks to a reduced flow velocity. This prevents e.g. 70 °C hot water from the solar system from mixing with 90 °C hot water in the uppermost area of the storage tank to reach e.g. 80 °C.
Sensible storage tanks use thermal insulation to maintain temperature and stratification and thus reduce heat loss. For smaller storage tanks in single-family homes (500-3,000 litres), this is achieved with a 10-20 cm thick layer of polyurethane foam or an equivalent thickness of mineral wool. For large seasonal district storages with several thousand cubic metres of capacity, able to supply an entire neighbourhood (see Figure 3), insulation is provided by an approximately 1 m thick layer of a suitable fill material, such as perlite.
Typical cooling rates for single-family home storages amount to a few degrees Celsius per day. They decrease as the heat capacity of the storage medium grows and the surface-to-volume ratio and insulation value (thermal conductivity in relation to the thickness of the insulation) shrink. Therefore, large, vacuum super-insulated (VSI) storage tanks (vakuumpufferspeicher.de) provide the best insulation, as seen in Figure 1. In them, the temperature drops approximately 5 times slower than in conventionally insulated tanks, making them ideally suited for long-term storage. With materials such as mineral wool, ageing and a consequent deterioration of the insulating effect may occur. Attention must be paid to proper installation, avoiding moisture penetration and thermal bridges. Therefore, cold storages are usually thermally insulated with closed-pore foam to prevent moisture penetration through condensation on the storage surface if the temperature of the storage medium falls below the dew point.
Location requirements: none
Focus on provision of power or energy: Energy
Suitable fields of application: water temperatures up to 100 °C (150 °C when pressurised)
Technology Readiness Level (TRL): 9
State of development/commercial availability: Using water as a storage medium: finished products, commercially available
Technical Specifications:
| Specific energy storage density | kWh/m³ | kWh/t |
| 60-100 | 60-100 | |
| Specific power density | kW/m³ | kW/t |
| 30-500 | 30-500 | |
| Typical/feasible storage size | MWhout | MWout |
| 0.03-1,000 | 0.015-1 | |
| System efficiency 2) | ||
| Storage efficiency | 50-90 | |
| Storage duration | hours-months | |
| Response time | minutes | |
| Service life (maximum) | Cycles | Years |
| 20-4,000 | ||
| Loss per time in % | 0.5 to 2.5 per day |
Notes on these specifications:
Economic Specifications:
Investment cost per kW: 1-15 €
Investment cost per kWh: 0.4-10 €
Operating and maintenance cost (based on investment/kW and kWh):
Note: Due to the numerous and strongly varying technical designs of such storage systems and with regard to the requirements of different fields of application, the costs in relation to performance and energy vary greatly. The cheapest sensible heat storages are currently used in Denmark for seasonal storage. The extremely simple design of very large storage tanks (lined outdoor pits covered with foil) can bring cost down to 0.35 € per kWh of installed storage capacity. Wherever water cannot be used for a storage medium and highly efficient insulation technologies are necessary, the costs rise accordingly.
See for further information:
- ZAE Bayern, https://en.zae-bayern.de/
