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Energy resources and their utilization intimately relate to sustainable development. In attaining sustainable development, increasing the energy efficiencies of processes utilizing sustainable energy resources plays an important role. The utilization of renewable energy offers a wide range of exceptional benefits. There is also a link between exergy and sustainable development. Exergy analysis has been widely used in the design, simulation, and performance evaluation of various energy systems as well as renewable energy sources. In this regard, determination of exergy of solar radiation is very crucial for various solar energy-related applications and is based on the relative potential of the maximum energy available from radiation. The efficiency factor limiting the gain of the maximum useful energy from the solar radiation is significantly similar to that of the Carnot efficiency for the heat engines. The main objectives of this study are two-fold, namely, (i) to comprehensively review various solar exergy models used in solar energy-related applications, and (ii) to determine the solar exergetic values for some regions of Saudi Arabia and Turkey, which are taken as two illustrative examples to which various models have been applied and compared. In this regard, the ratios of solar radiation exergy to solar radiation energy (exergy-to-energy ratio) for northeastern Saudi Arabia are calculated to be on average 0.933 for both approaches of Petela and Spanner and 0.950 for Jefer's approach at outside air temperatures between 16.18 and 33.01 ı C. These ratios for Izmir, Turkey are obtained to be on average 0.935 and 0.951 for the same approaches at a temperature range of 15-22 ı C, respectively. The values found using Jefer's approach appear to be 2% larger than the approaches of Petela and Spanner, while those are very close to the value of 0.95 proposed by Nobusawa.

Particle heating receivers are a promising technology that can allow operation of CSP systems at temperatures higher than what today's commercial molten salt systems can achieve, making them suitable for use in a variety of applications, including supercritical CO 2 cycles, air Brayton cycles, and high-temperature process heat. One of the ways to improve costcompetitiveness of particle heating receivers is to use low-cost particulate materials, such as sand, as the working medium. Red sand is particularly attractive due to its abundance and acceptable absorptance. This paper presents the results of on-sun testing of a particle heating receiver that uses red sand as the working medium. Tests were conducted at the experimental central receiver facility at King Saud University in Riyadh, Saudi Arabia. Performance of the receiver was assessed in two ways. First, the rate of thermal energy absorption was calculated using the measured temperature rise across the receiver, particle flow rate, and red sand's specific heat. Second, receiver efficiency was calculated using the rate of thermal energy absorption and the thermal power incident on the receiver, which was estimated using a raytracing software. Results show that a temperature rise of 130°C was achieved with an incident heat flux of 230-280 kW/m 2 . Receiver efficiency was found to range from 60% to 70%. These results are encouraging and show that red sand is a promising particulate material, especially when it is used with a proper cavity receiver design where the effect of absorptance of the particulate material becomes less significant.

This paper presents the results of an extensive study of two thermal energy storage (TES) systems. The goal of the research is to make solar energy cost-competitive with other forms of electricity. A small-scale TES system was first built. The inner to outer layers were made of firebrick (FB), autoclaved aerated concrete (AAC) and reinforced concrete brick (CB). The experiments were conducted at temperatures of up to 1000 °C for sustained periods of time. AAC was found to be prone to cracking at temperatures exceeding 900 °C; as a result, AAC was eliminated from the second TES system. The second, larger-scale TES system was subsequently built of multiple layers of readily available materials, namely, insulating firebrick (IFB), perlite concrete (PC), expansion joint (EJ), and CB. All of the surfaces were instrumented with thermocouples to estimate the heat loss from the system. OPEN ACCESSEnergies 2014, 7 8202The temperature was maintained at approximately 800 °C to approximate steady state conditions closely. The steady state heat loss was determined to be approximately 4.4% for a day. The results indicate that high-temperature TES systems can be constructed of readily available materials while meeting the heat loss requirements for a falling particle receiver system, thereby contributing to reducing the overall cost of concentrating solar power systems.