Long-term, compact thermal energy storage( TES) is essential to the development of cost-effective solar and passive building-integrated space heating systems. A paper presented at SHC 2013 demonstrates the ability of a geothermal heat exchanger with a 100m borehole tobe used as cold source for a closed adsorption seasonal storage for a “low-energy” building. According to the research the seasonal storage would allow for nearly 100% solar fraction to be reached for space heating. R718 is used as the adsorbate in this simulation, which was run using the softwareTRNSYS.

The paper, “Simulation of a vertical ground heat exchanger as low temperature heat source for a closed adsorption seasonal storage of solar heat ”by Samuel Hennaut, Sebastien Thomas, Elisabeth Davin, Alexandre Skrylnyk, MarcFrere, Philippe Andre, was presented during the 2nd International Conference on Solar Heating and Cooling for Buildings and Industry (SHC 2013) held on 23-25 September 2013 in Freiburg, Germany. Thermochemical or sorption seasonal heat storage is currently seen as a promising solution for drastically and efficiently increasing the solar fraction of solar combisystems in temperate climates.

Combisystem using closed adsorption system and seasonal storage allows 100% fractional energy savings

The total area of the solar collectors in this study is 20 m².These collectors heat a 1,5m³ tank used for space heating and domestic hot water (DHW). The building is only heated from October 1st to April 30th. During this period, the temperature in the tank is limited to 95°C and the seasonal storage is only discharged. During the second part of the year (“storage period”), the building isn’t heated and the temperature in the tank may not exceed 75°C. When this maximal temperature is reached in the tank, the excess solar heat is stored thermochemically.

The seasonal storage occurs through an adsorption closed system, composed of a reactor integrating a heat exchanger. The reactor includes the totality of the adsorbent and is connected to a tank containing the adsorbate. Adsorbent and adsorbate are separated by a valve, which controls the transfer of the adsorbate. A heat exchanger is included in the adsorbate tank.

The closed systems’ output is calculated to reach fractional thermal energy savings close to 100% at moderate collector and store sizes.

How does the closed adsorption system work?

The system is presented in Fig.1. in summer (a) and winter (b) operation mode.

Fig. 1. Closed adsorption system (a) charging mode; (b) discharging mode.

During charging phase heat supplied by solar collectors increases the temperature of the hydrated adsorbent leading to water vapour desorption. To avoid reaching the pressure equilibrium between water vapour and adsorbed water, the desorbed water has to be transferred to the tank (to continue the desorption reaction). This transfer occurs if the vapour pressure in the tankis lower than in the reactor. To achieve this lower vapour pressure in the tank, water vapour must be condensed in the tank.

The higher the condensation temperature is, the higher the temperature in the reactor has to be to achieve desorption.Therefore, to maintain the relatively low temperature and water pressure in the tank, a cold sink is necessary to evacuate the heat of condensation of water. In these simulations, the cold sink is the ground, which is heated through avertical ground heat exchanger (GHX).

During the discharging phase, water stored inthe tank has to be vaporised to react with the adsorbent. The heat of vaporisation of the sorbate is supplied by the cold source through the GHX. The temperature reached at the evaporator will determine the maximal vapour pressure in the tank, which has to be greater than in the reactor to ensure a flow of vapour to the latter.

The vapour pressure in the reactor will determine the maximal adsorption temperature, which directly influences the space heating supply temperature. So, the higher the evaporation temperature is, the higher the temperature is in the reactor and in the heating loop.

Future research

Further work is needed to study the influence of the depth on the ground temperature decrease and also on the parasitic consumption of this cold source. Indeed, the length of the pipe and the flowrate will influence directly the power consumption of the pump. The estimated annual consumption of the pump represents more than 10 % of the energy necessary for the building space heating.

The investment cost of such a borehole is also quite expensive. The GHX alone costs around 50 €/m (without working fluid,building connection and pump). As a conclusion, this cold source allows the use of free energy but not freely.

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