Latent heat thermal storage with PV for nearly zero-energy buildings


Researchers at Politecnico di Torino in Italy are testing how latent heat thermal storage (LHTS) could be used in combination with rooftop PV to achieve nearly zero-energy buildings (NZEB). NZEBS are residential and commercial buildings that have very low primary energy requirements, which have to be met to a significant extent by renewable sources.

“In the European project RE-cognition, we are using LHTS and PV independently of each other to meet the electrical and thermal demand of our pilot building – the Energy Center at our campus,” researcher Giulia Manco told pv magazine. “In general, however, it would be possible to couple photovoltaics to this type of heat storage if you had a heat pump capable of converting the electricity produced by photovoltaics into thermal energy.”

He said the configuration would facilitate the accumulation of surplus electricity from photovoltaics in the form of heat, thus increasing the self-consumption of renewable energy.

“A heat pump is currently not included in this specific project,” said researcher Alessandro Colangelo. “Therefore, the LHTS will allow increasing the flexibility of the thermal demand of the pilot building, while photovoltaics will increase the share of renewable electricity consumed on site.”

The 47kW PV system consists of a southwest-oriented solar array on the rooftop and several southeast-oriented PV panels integrated into the hall windows. The pilot building also has other renewable energy technologies such a geothermal heat pump, a district heating substation, and two small experimental wind turbines.

The latent heat thermal storage system is described by the scientists as able to accumulate significant amounts of thermal energy to use when heating is needed. The system is based on a special kind of phase change material (PCM) related to bio-based paraffin, with a melting point at 74 C.

PCMs are substances that absorb and release heat energy when they change phase and are able to store and release large amounts of latent heat over a defined temperature range. In the construction business, combining building materials with PCMs is an efficient way to improve the thermal energy storage capacity of construction elements. The cost of PCM is strongly dependent on the type of material.

“Organic PCMs are the easiest to use and cost between 2-10 €/kg,” Manco said. “The specific energy content is also rather variable, but approximately organic PCMs are able to store 200 kJ/kg”

The LHTS  appear as a tank filled with the PCM, crossed by several pipes where the water of the heating system is flowing.

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“To enhance the rate of the heat exchange between the water and the PCM, extensions made of high conductive material are added to the pipes, thus facilitating the charging and discharging processes,” the research group explained.

In the charging phase, the initially solid PCM melts gradually, heat is stored and hot water can be supplied through the pipes. In the discharging process, the initially liquid PCM solidifies when heat is released and cold water can be supplied through the same pipes.

“The energy required for these transformations is very high since the molecular structure of the PCM is modified. Hence, a large amount of heat can be accumulated in a relatively small volume, up to five times lower than a traditional water tank,” the academics said.

The proposed system was built with two tanks that can store up to 40kWh of thermal energy each and different internal pipe extensions.

“Following a topological optimization approach in the design phase, the same quantity of high conductive material was arranged differently around the tubes to increase the LHTS performance,” the scientists said.

The two storage tanks can purportedly release most of their energy content in less than one hour. Experimental tests are currently being made by the group, which said the real potential of this technology should be well known by this spring.

“It would be immediately transferable to commercial production and there is already a start-up here in Turin working on this type of devices,” Colantuono said.

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