Abstract
A typical solar domestic water heating system suffers from low energy efficiency due to multiple heat transfer process among components, i.e., the solar thermal collector and the thermal energy storage. In this work, a compact design of storage-integrated solar thermal collector and its compatible phase change material (PCM) storage material were explored. The salt hydrate based composite PCM, the sodium acetate trihydrate (SAT), was used to create a shape stabilized PCM (ssPCMs) for a feasible PCM by its low leakage and high thermal conductivity. The developed ssPCMs has been tested experimentally and a numerical model has been developed to obtain the effective thermal property of the developed ssPCMs by comparing the predicted results with the measurements. Furthermore, the model has been used to study the (dis)charging behavior of PCM in a storage-integrated solar thermal collector which is composed of an evacuated tube and a double spiral coils heat exchanger. A geometrical optimization has been performed to achieve 2.6 times longer of discharging period with a minimum outlet temperature of 55 °C. Moreover, the circulating water is found useful in increasing the PCM charging rate with 9% by transferring the heat from the surface to the center of tube.
Highlights
In Europe, more than 50% of the energy used in households is in the form of heat [1], and supplying domestic hot water is still the major application of solar thermal system
the water outlet temperature (Tout) decreases over time along with the phase change material (PCM) got discharged
The solar irradiance experiments with the lab-sized collectors identify the effectiveness of the chosen material, salt hydrate based composite PCM, sodium acetate trihydrate (SAT)-sodium dihydrogen phosphate monohydrate (SPM)-expanded graphite (EG) (SSE) on thermal energy charging and discharging
Summary
In Europe, more than 50% of the energy used in households is in the form of heat [1], and supplying domestic hot water is still the major application of solar thermal system. One of the most promising way to tackle this problem is using heat storage together with solar thermal collectors In this way the thermal storage unit stores the excess energy generated during the day and releases it when it is needed (i.e., during the night). Sensible heat storage is most widely used for daily and short-term thermal energy storage due to its technology maturity and low cost, e.g., hot water tanks. It cannot store heat for a longer period mainly due to the large heat loss. Latent heat storage (i.e., phase changer materials) offers higher energy density and smaller temperature variation during phase transition which allows storing larger quantity of heat in smaller volume and supply the heat with less heat loss [2,3]
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