MATERIALS AND METHODS: The integration of the LHS unit as an active component of the central heating system enables thermal buffering to support the heat pump. Depending on the heating demand in the building, the return temperature of the heating loop might be lower than 34 °C, and thus require additional power from the heat pump to sustain 40 °C as outlet temperature. Integrating the LHS unit downstream from the heat pump, with the option to circulate the return water through it or not, provides the opportunity to both charge and discharge the LHS unit, while smoothing the output demand from the heat pump. Charging the LHS unit occurs when the heating demand is low, using 40 °C as inlet temperature, as it is generated by the heat pump. Using a PCM with phase change temperature within 34-37 °C, return water at lower temperature than 34 °C can circulate through the charged LHS unit and be heated up before entering the heat pump. Additionally, the LHS unit can be directly charged using the district heating loop providing hot water at 47 °C. Another feature available with this integration is the opportunity to use the LHS unit as a direct heat source in the building heating loop. This is meant to occur when the LHS unit is charged and the heating demand in the building is relatively low. Therefore, the heat pump can be bypassed, reducing significantly the energy use during these low-demand periods. This operational mode is especially interesting if energy price is integrated in the control system of the overall heating system.
To be suitable to the application, the PCM should primarily have a melting temperature within 35-37 °C and limited supercooling. This limits the PCM selection to only a limited range of commercially available paraffin based PCMs. After investigation, the PCM CrodaTherm 37 (CT37) was selected due to its low degree of supercooling, its low-carbon footprint as well as its affordability.
The heat exchanger containing the PCM includes 24 laser-welded stainless-steel pillow-plates, mounted vertically in parallel with a 40-mm pitch. Water circulates in each of them following a 2-pass pattern. Headers are gathered at one end of the unit to enable a homogeneous distribution of the water across the pillow-plates. The heat exchanger is filled with ca. 3 tons of PCM CT37 to occupy the volume between the pillow plates. A thick mineral wool thermal insulation around the LHS-unit allows for a theoretical heat loss under 1 % per 24 h.
RESULTS AND CONCLUSION: The aim of the project is to design and integrate a LHS unit in the central heating system of the ZEB Laboratory building in Trondheim (Norway). The LHS unit is designed based onpillow-plate heat exchanger filled with PCM whose phase change temperature is 35-37 °C. The designed LHS unit can store up to 194 kWh heat and simultaneously achieve sufficiently high heat transfer rates during discharge to successfully back up the heat pump for 2-3 days during the coldest winter days or be used as a heat source in the central heating system. Installation has been completed and the first tests are on-going.