b Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA
In any solarthermal application, such as solar space heating, solar hot water for domestic or industrial use, concentrating solar power, or solar air conditioning, a solar receiver converts incident sunlight into heat. In order to be efficient, the receiver must ideally absorb the entire solar spectrum while losing relatively little heat. Currently, state-of-the-art receivers utilize a vacuum gap above an absorbing surface to minimize the convection losses, and selective surfaces to reduce radiative losses. Here we investigate a receiver design that utilizes aerogels to suppress radiation losses, boosting the efficiency of solarthermal conversion. We predict that receivers using aerogels could be more efficient than vacuum-gap receivers over a wide range of operating temperatures and optical concentrations. Aerogel-based receivers also make possible new geometries that cannot be achieved with vacuum-gap receivers.
There is a need to enhance the performance of Solar Power Tower (SPT) systems in view of their significant capital costs. In this context, the preliminary design step is of great interest as improvements here can reduce the global cost. This paper presents an optimization method that approaches optimal SPT system design through the coupling of a Particle Swarm Optimization algorithm and a Monte Carlo algorithm, in order to assess both the yearly heliostat field optical efficiency and the thermal energy collected annually by an SPT system. This global optimization approach is then validated on a well- known SPT system, ie the PS10 SolarThermal Power plant. First, the direct model is compared to in-situ measurements and simulation results. Then, the PS10 heliostat field is redesigned using the optimization tool. This redesign step leads to an annual gain between 3.34 % and 23.5 % in terms of the thermal energy collected and up to about 9 % in terms of the heliostat field optical efficiency from case to case.
Energies 2021, 14, 2146 2 of 24
TIR based micro-optics solar concentrator, the incoming sunlight is focused by a micro-lens onto one or more localized scatterers in the waveguide, which guides the light waves towards the periphery via TIR, where the concentrated radiation is converted to heat on the surface of the receiver tube and transferred to the flowing heat transfer fluid. The cost effectiveness of the waveguide solarthermal concentration approach is primarily attributed to the reduction or exclusion of tracking cost that makes the system compact, and the flat planar form factor that enhances land-use efficiency as the collectors can be spaced closely (with no shading concerns), leading to increased power generation capability for a given land area. The light weight, simple installation protocol and structural design due to the flat planar form factor could leverage advances in the photovoltaics market for low-cost installation. Furthermore, the waveguides could be placed closer to ground, thereby elimi- nating or minimizing the heavy metallic structures that are currently needed to support troughs and heliostats against wind loading.
Solarthermal, solar thermoelectrics, and solar thermophotovoltaics (TPV) offer three poten- tially high-efficiency paths for converting sunlight into electricity. All three ideally absorb sun- light strongly but have low thermal reradiation – a combination known as a selective solar absorber . The heat can then be either used directly, or used to drive an electrical generator. In the most traditional case, heat is driven into a working fluid to run a mechanical engine . Solar thermoelectrics instead use the Seebeck effect to generate electricity across a thermal gradient [3, 4]. In the case of solar TPV, as illustrated in Fig. 1, the selective absorber is ther- mally coupled to a selective emitter, which thermally radiates onto a nearby TPV cell capable of converting photons above the TPV bandgap energy directly into electricity [5–8]. The ad- vantage of these approaches over traditional solar photovoltaics (PV) is that they can avoid two major sources of PV loss: thermalization of high-energy photons and reflection of low- energy photons. By absorbing almost all incoming solar photons as heat, and only re-radiating a small amount, the overall system efficiencies can approach the Carnot limit . However, experimental systems have fallen well short of this ideal. A substantial amount of loss has been observed to occur both in selective solar absorbers as well as selective emitters, particularly under conditions of low concentration or high operating temperatures .
a Département de Physique, Faculté des Sciences Exactes et Sciences de la Nature et la Vie, Université Larbi BenM’hidi, Oum El Bouaghi, ALGERIA b Département de Génie Climatique, Faculté des Sciences de l’Ingénieur, Université Frères Mentouri, Constantine, ALGERIA
The objective of this work is to study the feasibility and performances of solarthermal energy in the region of Constantine, situated at the north of Algeria and if it is profitable to a farmer to use it. The air solar collector is made with cheap available materials and has given important results with an air heated to more than 60 ºC, with an optimum surface of 3 m 2 , inclined with 10 degrees and directed to the south. Also, coupling the air collector with a drying chamber has given other satisfactory results. It was found that solar drying is influenced by the collector parameters in particular its surface, by the exterior ambient conditions such as the velocity and the temperature of the ambient air and also by the dimensions of the agro-alimentary dried product. Adding a heater allow the use of the solar system in unfavorable climatic conditions.
Solar absorber consisting of a metallic substrate coated with a high solar absorption coefficient film is a critical component of the solarthermal system. Several techniques are used to produce these coating films. Starting from the raw materials, a nano- powder formulation is produced to suite the film characteristics and the coating technique requirements. Once optimized and tested, the coated substrate is packaged and integrated within the overall solarthermal system. The overall value chain consist- ing of the five main extraction, manufacturing and installations steps is summarized in Scheme 1. In this article, we will focus mainly in the third step (coating film formation), although Step 2 (formulation) and Step 4 (optimization and testing) will be also discussed briefly.
1.1 Concentrated Solar Power
Concentrated Solar Power (CSP) systems harness the energy of the sun using a reflective surface to concentrate direct solar irradiation of a large area onto a smaller receptive area, in order to generate high temperatures, and therefore high heat flux . The heat thereby generated can be used to drive a variety of processes. In coastal regions with low fresh water availability, CSP technologies are explored to provide the energy for the desalination of sea water . In other cases, it is used to facilitate the extraction of oil by using solar energy to heat up underground deposits and increase pressure, a process referred to as “Solarthermal enhanced oil recovery” . But most importantly, CSP can be used for power generation , by driving a regular thermodynamic cycle, sunlight being used as the heat source instead of the burning of a combustible that would typically be either nuclear material or fossil fuel. The current work focuses solely on the power generation application.
The various approaches, described in more detail in Chapter 7, include evolutionary and gradient methods, among others. The genetic algorithm (GA), inspired by biolog- ical evolution, “breeds” a population of ever more optimized solutions through succes- sive generations, escaping the local maxima or minima that can trap gradient-ascent ( “hill-climbing”) methods via “mutations” that sample more widely in the search space. Particle swarm optimization (PSO) similarly employs a population of solutions, with the candidates initially dispersed and the vector of the “swarm” in the search space is parametrically updated to move towards the “leader” (optimum metric for a given search timestep) without reference to any evolutionary operators such as “breeding”, “crossover”, or “mutation.” Once the leader is established in the mono- tonic region of the global optimum, the optimization can drop the swarm and switch to a faster gradient-ascent/descent from the leader ’s position, however, this requires either some understanding of the search space or a metric for identi ﬁcation of the globally optimum ‘hill.’ GA and PSO, as well as more traditional gradient methods, can be applied to solar ORC optimization to greatly reduce computation time; for deeper analysis into their relative bene ﬁts the reader is referred to the literature ( Hassan et al., 2004 ).
Fig. 1 . The loop is composed of a hot storage tank connected to the exit of the solar receiver, which feeds a ﬂuid bed heat exchanger (FBHE), where the particles transmit their energy to submerged tubes inside whose working ﬂuid (for example steam) is gener- ated, the latter is then expanded in a turbine. FBHE is indeed a classical device in the electrical power industry (mostly imple- mented for coal combustion in ﬂuidized bed). The cooled particles exit the exchanger (continuous circulation) and are sent towards the cold storage tank; this can be done either by mechanical or pneumatic conveying or by gravity depending on the available space or on the facility geometry (tower conﬁguration is particu- larly favourable for gravity for instance). Finally, connecting the cold bin to the solar receiver inlet by a conveying system raising the particles completes the loop. Consequently, solid particles are used as heat transfer ﬂuid and heat storage medium. Actually, it should be noted that the proposed solar power plant is combined with a vapour cycle and steam turbine, but the system is very similar to the case of a gas turbine, the main difference being the heat exchanger, which is changed to adapt to the chosen type of turbine. In this concept the particle solar receiver is the key component. The next paragraph summarizes the state-of-the-art in the ﬁeld of solar receivers using particles as HTF.
Sapkota et al.  examine the impacts of the use of some RE (such as biogas, improved cooking stoves, micro hydro and solar power) in rural communities in Nepal. This study uses the Long-range Energy Alternatives Planning (LEAP) model. The results of this model show that the implementation of micro hydro for the next 20 years will reduce CO2 emissions by 2.553 million tons (Mt). Concerning the use of solar power, biogas and improved cooking stoves, it will permit a significant reduction in CO2 emissions of 5.214 Mt, 35.880 Mt and 7.452 Mt, respectively. Shafiei and Salim  try to explore the causes of CO2 emissions. They use a STIRPAT model based on data from OECD countries (from 1980 to 2011). The empirical results show that renewable energy consumption decreases CO2 emissions whereas conventional energy consumption increases CO2 emissions. Yadoo and Cruickshank  used sustainability indicators in order to explore the role of renewable energy mini-grids (in Nepal, Peru and Kenya) in climate change mitigation and poverty reduction.
Oils are important HTFs for solar-thermal applications since they offer the best available combination of low freezing point and high upper temperature limit. Table 5 shows the temperature range and thermo- physical properties of some commercially available mineral and synthetic oil based heat transfer fluids. These HTFs are liquid at ambient conditions and do not require external temperature control to maintain a reasonably low viscosity. Synthetic oils (e.g. Therminol ® VP-1, Solutia Inc.) are common in parabolic trough solar plants but are ultimately limited by their relatively low operating temperature of about 393 °C. While in principle the HTF performance improves with increasing temperature (due to the reduction in the liquid’s viscosity and therefore required pumping power), solar engineers are typically faced with stability issues which ultimately sets the operating temperature and maximum attainable exergetic efficiency (see Section 5). Some recent studies suggest that biphenyl- and diphenyl- oxide based thermal fluids such as Therminol VP-1 and Dowtherm A undergo gradual thermal decomposition at temperatures close to 400 °C. 209 This gradual thermal breakdown results in hydrogen gas formation that permeates through steel tubes into the vacuum enclosure and increases the heat loss. A pressurization system and a nitrogen blanket is thus provided to prevent air from contacting the hot oil which could lead to performance degradation due to oxidation and increased flammability hazard. However, these preventive mechanisms increase the operation cost significantly.
The three designs analysed were based on thermal performance of PV-T modules. In all three cases the cooling water enters at first cell and exits after flowing over many cells to collect heat. It clearly shows, all solar cells along the flow path will have non-uniform temperature that will cause electrical mismatching. Moreover, solar cells on either side of junction box shown in black at the top are not even cooled and will operate at higher temperature. These designs will have maximum electrical mismatch between the first cell which receives the coldest water for cooling to last cell where the warm water exists. All cooling pipe configurations are commonly used in solarthermal collectors. The primary aim is to recover heat and increase overall efficiency of PV-T module. They have not address the important issue of mismatching which is important for PV modules. López, et al.,  developed an innovative low-cost prototype of a PV-T module using pipes of a floor heating system filled with a special brine to cool the PV cells. The comparative performance of PV-T versus identical uncooled PV module of 130W is shown in Figure 2-15. The daylong testing from 8:30PM to 7PM on a clear sunny day showed 200 litres cold water temperature increased from 20°C to 24°C. A 6% increase in the electrical output in watt-hour was achieved though it is not as high as expected.
Bikram Bhatia, 1 Lin Zhao, 1 Yi Huang, 1 James Loomis, 1 Feng Cao, 2 Svetlana V. Boriskina, 1 Zhifeng Ren, 2 Evelyn N. Wang, 1 and Gang Chen* 1
Growth of renewable electricity generation is critical to reducing the pace of global climate change. Solar energy offers a promising renewable energy source, however it is expensive to store electricity from photovoltaics (PV), the most widely deployed solar electricity technology. Solarthermal energy is another solar technology that can be paired with inexpensive thermal storage, increasing the dispatchability of the generated electricity, but solarthermal systems are more expensive overall. We have developed a solar receiver that combines PV and solarthermal to efficiently convert solar radiation to electricity (to be used immediately) and thermal energy (to be stored and converted to electricity on demand). This paper describes the Hybrid Electric And ThermalSolar (HEATS) receiver and models its performance. An idealized model indicates that the HEATS receiver is capable of achieving high solar to electricity efficiency (35.2%) with high dispatchability (44.2% of electricity from thermal energy) at an operating temperature of 775K. Modeling using measured performance values for the HEATS subcomponents predicts an efficiency of 26.8% with a dispatchability of 81% with a silicon PV cell and 28.5% efficiency and 76% dispatchability with a gallium arsenide PV cell, with both configurations using an operating temperature of 700K. Measured effective transmittance through a preliminary prototype HEATS receiver agrees well with modeled values, which demonstrates the feasibility of the concept.
It is essential to verify the thermal design of the instrument, especially the heat evacuation property and to assess the thermo-mechanical behavior of the instrument when submitted to high thermal load.
Therefore, a thermal balance test under 13 solar constants was performed on the first model of EUI, the Structural and Thermal Model. The optical analyses and experiments performed to characterize accurately the thermal and divergence parameters of the flux are presented; the set-up of the test, and the correlation with the thermal model performed to deduce the unknown thermal parameters of the instrument and assess its temperature profile under real flight conditions are also presented.