Parametric study of a thermal trap solar energy collector

A parametric study of a thermal trap solar energy collector with the help of a modified Hottel-Whillier-Bliss equation, is presented. The developed analysis is used to optimize the typical parameters, namely the trap's thickness and the number of flowing channels. The variation of the rating parameters of a collector with typical quantities, such as the fin distance, mass flow rate and thickness of the absorber plate, is discussed in detail.     

A bilinear model for the thermal behaviour of a point focusing solar collector

An important task when designing point focusing (paraboloidal) solar-collectors is to model the thermal behaviour of their receivers by an approximate dynamic model in view of thermal loop automatization. In general, this is very difficult or impossible by studying the exact physical phenomena which occur in the receiver. However, approximations can be made and approximate models can always be derived. Using filtering and estimation theory these models can be identified to represent reality very well.     

On increasing solar gains through roof/wall with a thermal trap

This paper investigates the feasibility of increasing solar gains through the roof/wall of a building utilizing the thermal trap effect. It is seen that optimum thickness of the thermal trap is 0.05m. Corresponding to this thickness, there is a net positive heat gain into the room at all times for a 0.20m thickness of concrete wall/roof.     

Thermal restoration of a wall after the interruption of solar radiation flux

The transient phenomenon of the thermal restoration of an opaque wall of a room after the interruption of solar radiation flux was analysed. The wall, was initially, in thermal equilibrium under solar radiation. When the solar radiation flux was interrupted, the temperature distribution on the wall cross-section went through a transient state until it reached a final equilibrium state. During the thermal restoration of the wall, a thermal gain is maintained for the room as a result of the exploitation of a part of the thermal energy stored in the wall. Analytical expressions have been obtained for the duration of the thermal restoration, for the thermal gain of the room during this transient state and for the coefficients of storage and exploitation of the solar radiation by the wall.     

Explicit model of photovoltaic panels to determine voltages and currents at the maximum power point

A simple explicit photovoltaic formulation for characterizing and dimensioning cell-arrays is presented. The method permits the short-circuit current, the open-circuit voltage, the maximum cell power and the optimum cell-operation conditions to be determined. Further, the model also allows quantifying the effects of panel temperature and solar irradiance on key cell parameters. Based on several datasheets, the methodology is validated by covering a wide range of operation conditions. The proposed approach can thus, be very useful for design engineers to quickly and easily determine the performance of any photovoltaic array without performing tedious numerical calculations.     

Improved photovoltaic energy output for cloudy conditions with a solar tracking system

This work describes measurements of the solar irradiance made during cloudy periods in order to improve the amount of solar energy captured during such periods. It is well-known that 2-axis tracking, in which solar modules are pointed at the sun, improves the overall capture of solar energy by a given area of modules by 30-50% versus modules with a fixed tilt. On sunny days the direct sunshine accounts for up to 90% of the total solar energy, with the other 10% from diffuse (scattered) solar energy. However, during overcast conditions nearly all of the solar irradiance is diffuse radiation that is isotropically-distributed over the whole sky. An analysis of our data shows that during overcast conditions, tilting a solar module or sensor away from the zenith reduces the irradiance relative to a horizontal configuration, in which the sensor or module is pointed toward the zenith (horizontal module tilt), and thus receives the highest amount of this isotropically-distributed sky radiation. This observation led to an improved tracking algorithm in which a solar array would track the sun during cloud-free periods using 2-axis tracking, when the solar disk is visible, but go to a horizontal configuration when the sky becomes overcast. During cloudy periods we show that a horizontal module orientation increases the solar energy capture by nearly 50% compared to 2-axis solar tracking during the same period. Improving the harvesting of solar energy on cloudy days is important to using solar energy on a daily basis for fueling fuel-cell electric vehicles or charging extended-range electric vehicles because it improves the energy capture on the days with the lowest hydrogen generation, which in turn reduces the system size and cost.     

多孔太阳墙式太阳能房间与普通房间热舒适性分析

通过对某多孔太阳墙式太阳能房间和普通房间的数值模拟,分析太阳墙板内空气的受热过程及流动情况,同时系统地比较两种建筑在工作区范围内的热舒适性。与普通采暖房间相比,在有多孔太阳墙送风的情况下,可将室外空气由-3℃加热到21℃,空气流动速度由0.1m/s提高到0.31m/s,提供44.6m3/h的新风量,一定程度上解决了冬季开窗换气所引起的室内热负荷及采暖负荷,论证了多孔太阳墙可使室内热舒适性明显增强的结论。     

Functional requirements for component films in a solar thin-film photovoltaic/thermal panel

The functional requirements of the component films of a solar thin-film photovoltaic/thermal panel were considered. Particular emphasis was placed on the new functions, that each layer is required to perform, in addition to their pre-existing functions. The cut-off wavelength of the window layer, required for solar selectivity, can be achieved with charge carrier concentrations typical of photovoltaic devices, and thus does not compromise electrical efficiency. The upper (semiconductor) absorber layer has a sufficiently high thermal conductivity that there is negligible temperature difference across the film, and thus negligible loss in thermal performance. The lower (cermet) absorber layer can be fabricated with a high ceramic content, to maintain high solar selectivity, without significant increase in electrical resistance. A thin layer of molybdenum-based cermet at the top of this layer can provide an Ohmic contact to the upper absorber layer. A layer of aluminium nitride between the metal substrate and the back metal contact can provide electrical isolation to avoid short-circuiting of series-connected cells, while maintaining a thermal path to the metal substrate and heat extraction systems. Potential problems of differential contraction of heated films and substrates were identified, with a recommendation that fabrication processes, which avoid heating, are preferable.     

Thermal modeling of a combined system of photovoltaic thermal (PV/T) solar water heater

In this paper, an integrated combined system of a photovoltaic (glass–glass) thermal (PV/T) solar water heater of capacity 200 l has been designed and tested in outdoor condition for composite climate of New Delhi. An analytical expression for characteristic equation for photovoltaic thermal (PV/T) flat plate collector has been derived for different condition as a function of design and climatic parameters. The testing of collector and system were carried out during February–April, 2007. It is observed that the photovoltaic thermal (PV/T) flat plate collector partially covered with PV module gives better thermal and average cell efficiency which is in accordance with the results reported by earlier researchers.     

Feed-in tariff for solar photovoltaic: The rise of Japan

Japan started implementing a national Feed-In Tariff (FiT) mechanism on the 1st July 2012, which included specific payment tariffs for solar photovoltaic (PV) installations. This marks a new era in the renewable energy landscape in Japan. This paper aims at analysing the solar PV prospect in Japan, particularly in both residential and non-residential sectors. The paper presents, first, an overview of energy trends in Japan prior to the Fukushima event. This is followed by a short review of solar PV progress in the country, highlighting the major policies and programmes that have been implemented as well as the installations that have been carried out over the past two decades. Next, the financial impact of the new FiT scheme on consumers is evaluated. The financial analysis investigates the total profit, the average annual return on investment and the payback period. For a comparison purposes, a similar financial analysis is also conducted with selected countries around the world – namely Germany, Italy and the United Kingdom. The results from this analysis indicate that the new Japanese FiT rate generates a good profit, a moderate annual return on investment and an acceptable payback period, suggesting an increasing trend of solar PV uptake over the next years.     

Thermal performance of a south facing wall as solar collector storage system

This paper illustrates an analysis of the performance of a solar wall as a collector storage system. the south facing wall consists of a mass of concrete/brick whose one surface is blackened and glazed, and a network of pipes (metallic or plastic) is laid in a plane from which heat can be extracted by flow of fluid in such a manner that the temperature of the plane of heat retrieval keeps constant. the collection efficiency of the system is found to be 80·0 and 60·7 per cent for collection temperatures 20 and 25°C respectively on the surface; the maxima/minima of the rate of heat retrieval (Q(t)) occurs about 12 h after the maxima/minima of solar temperature at a depth of the plane of heat retrieval = π/α1, with a maximum efficiency of 21·47.     

Performance of a building integrated photovoltaic/thermal (BIPVT) solar collector

The idea of combining photovoltaic and solar thermal collectors (PVT collectors) to provide electrical and heat energy is an area that has, until recently, received only limited attention. Although PVTs are not as prevalent as solar thermal systems, the integration of photovoltaic and solar thermal collectors into the walls or roofing structure of a building could provide greater opportunity for the use of renewable solar energy technologies. In this study, the design of a novel building integrated photovoltaic/thermal (BIPVT) solar collector was theoretically analysed through the use of a modified Hottel-Whillier model and was validated with experimental data from testing on a prototype BIPVT collector.The results showed that key design parameters such as the fin efficiency, the thermal conductivity between the PV cells and their supporting structure, and the lamination method had a significant influence on both the electrical and thermal efficiency of the BIPVT. Furthermore, it was shown that the BIPVT could be made of lower cost materials, such as pre-coated colour steel, without significant decreases in efficiency.Finally, it was shown that by integrating the BIPVT into the building rather than onto the building could result in a lower cost system. This was illustrated by the finding that insulating the rear of the BIPVT may be unnecessary when it is integrated into a roof above an enclosed air filled attic, as this air space acts as a passive insulating barrier.     

Searching for public benefits in solar subsidies: A case study on the Australian government’s residential photovoltaic rebate program

The Australian Government ran a renewable energy program in the 2000s that provided rebates to householders who acquired solar Photovoltaic (PV) energy systems. Originally called the Photovoltaic Rebate Program (PVRP), it was rebranded the Solar Homes and Communities Plan (SHCP) in November 2007. This paper evaluates both the PVRP and SHCP using measures of cost-effectiveness and fairness. It finds that the program was a major driver of a more than six-fold increase in PV generation capacity in the 2000s, albeit off a low base. In 2010, solar PV’s share of the Australian electricity market was still only 0.1%. The program was also environmentally ineffective and costly, reducing emissions by 0.09 MtCO₂-e/yr over the life of the rebated PV systems at an average cost of between AU$238 and AU$282/tCO₂-e. In addition, the data suggest there were equity issues associated with the program, with 66% of all successful applicants residing in postal areas that were rated as medium–high or high on a Socio-economic Status (SES) scale.     

Perspectives for solar thermal applications in Taiwan

Taiwan has long depended on imported fossil energy. The government is thus actively promoting the use of renewable energy. Since 2000, domestic installations of solar water heaters have increased substantially because of the long-term subsidies provided for such systems. However, data on the annual installation area of solar collectors in recent years indicated that the solar thermal industry in Taiwan has reached a bottleneck. The long-term policy providing subsidies must thus be revised. It is proposed that future thermal applications in Taiwan should focus on building-integrated solar thermal, photovoltaic/thermal, and industrial heating processes. Regarding building-integrated solar thermal systems, the current subsidy model can be continued (according to area of solar collectors); nevertheless, the application of photovoltaic/thermal and industrial heating systems must be determined according to the thermal output of such systems.     

Enhancement of solar gains through a roof using a thermal trap

This paper investigates the viability of increasing solar gains through the roof of an air-conditioned room using the thermal trap effect. In solving the heat conduction equations through the thermal trap and the concrete slab, the finite difference method has been employed and the initial conditions are derived from the assumption that, initially, the ceiling of the room and the top of the trap material are in equilibrium with constant room air temperature and the ambient air temperature, respectively. The effect of the thickness of the thermal trap and that of the concrete slab on the thermal flux transferred through the roof have been studied.     

A review on photovoltaic/thermal hybrid solar technology

A significant amount of research and development work on the photovoltaic/thermal (PVT) technology has been done since the 1970s. Many innovative systems and products have been put forward and their quality evaluated by academics and professionals. A range of theoretical models has been introduced and their appropriateness validated by experimental data. Important design parameters are identified. Collaborations have been underway amongst institutions or countries, helping to sort out the suitable products and systems with the best marketing potential. This article gives a review of the trend of development of the technology, in particular the advancements in recent years and the future work required.     

Analytical and quasi-explicit four arbitrary point method for extraction of solar cell single-diode model parameters

In this paper, the five parameters of the solar cell single-diode model are analytical and quasi-explicitly extracted for the first time, just using the coordinates of four arbitrary points of the characteristic I–V curve and the slopes of the curve in these points. The new method presented, called Analytical and Quasi-Explicit (AQE) method, is exact because no simplifications of the model nor a priori approximations of the parameters are used and, it is quasi-explicit in the sense that all the parameters except one are explicitly given. The unique parameter not explicitly computed is easily obtained by solving a five-degree polynomial equation. Accurate and practical conditions are provided to select which solution of the previous equation is the desired parameter.It is also introduced a very easy method to obtain, directly from real data measurements, the needed four points of the I–V curve as well as the slopes in these points, without using any kind of sophisticated techniques.Finally, some experimental results are presented to demonstrate the high accuracy and simplicity of the new method. The results are compared with the well-known analytical five-point method and the recent oblique asymptote method.     

A high-irradiance solar furnace for photovoltaic characterization and nanomaterial synthesis

A high-irradiance solar furnace geared toward (a) elucidating the distinctive physics of concentrator photovoltaics and (b) driving high-temperature reactors for the generation of novel nanostructures is described, with a target irradiance up to 12 W/mm². The opto-mechanical design permits real-sun flash illumination at a millisecond time scale so that solar cells can be characterized with only insubstantial increases in cell temperature even at irradiance levels of thousands of suns.     

Performance model for parabolic trough solar thermal power plants with thermal storage: Comparison to operating plant data

This paper describes a simulation model that reproduces the performance of parabolic trough solar thermal power plants with a thermal storage system. The aim of this model is to facilitate the prediction of the electricity output of these plants during the various stages of their planning, design, construction and operation. Model results for a 50 MW_e power plant are presented and compared to real data from an equivalent power plant currently operated by the ACS Industrial Group in Spain.     

Increasing the solar photovoltaic energy capture on sunny and cloudy days

This report analyzes an extensive set of measurements of the solar irradiance made using four identical solar arrays and associated solar sensors (collectively referred to as solar collectors) with different tilt angles relative to the earth’s surface, and thus the position of the sun, in order to determine an optimal tracking algorithm for capturing solar radiation. The study included a variety of ambient conditions including different seasons and both cloudy and cloud-free conditions. One set of solar collectors was always approximately pointed directly toward the sun (DTS) for a period around solar noon. These solar collectors thus captured the direct beam component of the solar radiation that predominates on sunny days. We found that on sunny days, solar collectors with a DTS configuration captured more solar energy in accordance with the well-known cosine dependence for the response of a flat-surfaced solar collector to the angle of incidence with direct beam radiation. In particular, a DTS orientation was found to capture up to twice as much solar energy as a horizontal (H) orientation in which the array is tilted toward the zenith. Another set of solar collectors always had an H orientation, and this best captured the diffuse component of the solar radiation that predominates on cloudy days. The dependence of the H/DTS ratio on the solar-collector tilt angle was in approximate agreement with the Isotropic Diffuse Model derived for heavily overcast conditions. During cloudy periods, we found that an H configuration increased the solar energy capture by nearly 40% compared to a DTS configuration during the same period, and we estimate the solar energy increase of an H configuration over a system that tracks the obscured solar disk could reach 50% over a whole heavily-overcast day. On an annual basis the increase is predicted to be much less, typically only about 1%, because the contribution of cloudy days to the total annual solar energy captured by a photovoltaic system is small. These results are consistent with the solar tracking algorithm optimized for cloudy conditions that we proposed in an earlier report and that was based on a much smaller data set. Improving the harvesting of solar energy on cloudy days deserves wider attention due to increasing efforts to utilize renewable solar energy. In particular, increasing the output of distributed solar power systems on cloudy days is important to developing solar-powered home fueling and charging systems for hydrogen-powered fuel-cell electric and battery-powered vehicles, respectively, because it reduces the system size and cost for solar power systems that are designed to have sufficient energy output on the worst (cloudy) days.