Total absorption and emission coefficients of selective solar absorbers are measured under high vacuum conditions from room temperature up to stagnation temperature. The sample under investigation is illuminated under vacuum @1000W/m and the sample temperature is recorded during heat up, equilibrium and cool down. During stagnation, the absorber temperature exceeds 300°C without concentration. Data analysis allows evaluating the solar absorptance and thermal emittance at different temperatures. These in turn are useful to predict evacuated solar panel performances at operating conditions.

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In this article the development of a high performance, double-wall vacuum insulated hot water thermal energy storage for high temperature applications is presented. In this concept, the main heat losses of the tank are limited to radiation and to the thermal bridges present in the wall of the tank and fittings. This concept is well suited for high temperature applications such as those found in the industrial sector where storage energy losses are an important issue. Few studies on double wall evacuated tanks were found in the open literature and none fully employed a completely evacuated gap as proposed in this study. A structural analysis was performed to validate the proposed design and ensure conformity to high temperature applications. Heat transfer calculations assessed the impact of low emissivity coatings on the radiative heat transport in the evacuated gap. An evaluation of the investment cost of the novel concept was also performed and comparisons made with conventional insulated TES on the market. A numerical model of the tank was developed and the thermal behaviour investigated under different configurations. An economic analysis presented the investment attractiveness with respect to the common TES alternatives on the market. Overall, the presented concept is clearly viable not only in terms of technical feasibility but also in terms of economic practicality.

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Among solar thermal collectors, the evacuated flat panel is emerging as a reference technology for operation at higher temperatures of up to 200 °C with an increased annual energy production owing to both direct and diffuse light capture. Accurate measurements of the optical properties of the selective absorbers used in such devices are key for a reliable estimation of the overall performance. These optical properties must be measured under high vacuum at high temperatures, conditions under which the panels are meant to operate. In this study, we accurately measured these properties using a calorimetric technique. The measurement procedure is based on a power balance equation for a flat sample suspended in a high-vacuum chamber with minimal thermal losses and is well adapted for this class of devices. Calorimetric measurements obtained under Sun and LED light revealed excellent reproducibility and good agreement with those obtained using traditional optical analysis at low temperatures in air. When extended up to the absorber stagnation temperature, which often exceeds 300 °C, the calorimetric measurements started to deviate from the optical measurements, indicating the importance of measuring under the operating conditions.

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