A comprehensive review of solar facades. Opaque solar facades

In antiquity, people already knew the principles of solar architecture, designing their houses to the south to take advantage of the sun in all seasons. Today, solar architecture is undergoing a true revolution because of the development, among other things, of special facades involved in the processes of heating, ventilation, thermal isolation, shading, electricity generation and lighting of homes: these are called “solar facades”. This paper aims to review the remarkable developments that have occurred during the first decade of this century in this field.     

A comprehensive review of solar facades. Transparent and translucent solar facades

In the first paper of a series of two publications entitled “A comprehensive review of solar facades. Opaque solar facades” an exhaustive review of scientific studies carried out during the last decade on opaque solar facades was proposed. The paper dealt with facades that absorb and reflect the incident solar radiation but cannot transfer directly solar heat gain into the building. This article offers a complementary survey of studies conducted during the same period of time on transparent and translucent solar facades, highlighting the categories of ventilated facades and semi-transparent building-integrated photovoltaic facades.     

Daylight, sunlight and solar gain in the urban environment

In cities, site layout has a substantial impact on the daylight, sunlight and solar radiation received by windows. This paper discusses ways to ensure solar access in obstructed situations, both within new developments and in existing buildings nearby.     

Photovoltaic-integrated building facades

Photovoltaic (PV) cells, when integrated within a building facade, offer the possibility of generating electric power and heat for local use or export. This paper reports on a project to investigate the practical operational efficiencies that might be delivered from such facades. The results from laboratory experiments and computer simulations are presented: the former were used to develop an empirical relationship between cell temperature and power output; the latter were undertaken to assess operational efficiencies under a range of climate conditions representative of the UK.     

Design optimization of solar cooker

Various designs of solar cookers have been theoretically investigated with a view to optimize their performance. Starting from a conventional box type cooker, various combinations of booster mirrors have been studied to arrive at a final design, aimed at providing a cooker, which can be fixed on a south facing window (for countries of northern hemisphere, mainly situated near the tropic of Cancer). This cooker, with a rear window opening, may provide higher cooking temperature for a fairly large duration of the day. Two or three changes in positions of the side booster mirrors, without moving the cooker as a whole has been proposed. The new design has been experimentally implemented and compared with a conventional box type solar cooker. Besides the convenience of a rear window opening, the cooker provides temperatures sufficiently high to enable cooking two meals a day.     

Performance of a multifunctional PV/T hybrid solar window

A building-integrated multifunctional PV/T solar window has been developed and evaluated. It is constructed of PV cells laminated on solar absorbers placed in a window behind the glazing. To reduce the cost of the solar electricity, tiltable reflectors have been introduced in the construction to focus radiation onto the solar cells. The reflectors render the possibility of controlling the amount of radiation transmitted into the building. The insulated reflectors also reduce the thermal losses through the window. A model for simulation of the electric and hot water production was developed. The model can perform yearly energy simulations where different features such as shading of the cells or effects of the glazing can be included or excluded. The simulation can be run with the reflectors in an active, up right, position or in a passive, horizontal, position. The simulation program was calibrated against measurements on a prototype solar window placed in Lund in the south of Sweden and against a solar window built into a single family house, Solgården, in Älvkarleö in the central part of Sweden. The results from the simulation shows that the solar window annually produces about 35% more electric energy per unit cell area compared to a vertical flat PV module.     

System analysis of a multifunctional PV/T hybrid solar window

The work presented in this article aims to investigate a PV/T hybrid solar window on a system level. A PV/T hybrid is an absorber on which solar cells have been laminated. The solar window is a PV/T hybrid collector with tiltable insulated reflectors integrated into a window. It simultaneously replaces thermal collectors, PV-modules and sunshade. The building integration lowers the total price of the construction since the collector utilizes the frame and the glazing in the window. When it is placed in the window a complex interaction takes place. On the positive side is the reduction of the thermal losses due to the insulated reflectors. On the negative side is the blocking of solar radiation that would otherwise heat the building passively. This limits the performance of the solar window since a photon can only be used once. To investigate the sum of such complex interaction a system analysis has to be performed. In this paper results are presented from such a system analysis showing both benefits and problems with the product. The building system with individual solar energy components, i.e. solar collector and PV modules, of the same size as the solar window, uses 1100 kW h less auxiliary energy than the system with a solar window. However, the solar window system uses 600 kW h less auxiliary energy than a system with no solar collector.     

Thermodynamic optimization of solar flat-plate collector

Second-law modeling of flat-plate collectors is performed with the objective of describing collector performance when entropy generation is minimum. Effects of irreversibility are demonstrated. Factors mostly effecting the generation of entropy are discussed.     

Size optimization of conventional solar collectors

An expression for the optimum length of a flat-plate solar collector that maximizes the life-cycle savings of the collector is derived. An expression has been obtained also for the optimal distribution of a finite amount of thermal insulation that minimizes the energy loss from the back side of a flat-plate solar collector.     

Thermo-ecological optimization of a solar collector

The depletion of non-renewable natural exergy resources (the thermo-ecological cost) has been accepted as the objective function for thermo-ecological optimization. Its general formulation has been cited. A detailed form of the objective function has been formulated for a solar collector producing hot water for household needs. The following design parameters have been accepted as the decision variables: the collector area per unit of the heat demand, the diameter of collector pipes, the distance of the pipe axes in the collector plate. The design parameters of the internal installation (the pipes, the hot water receiver) have not been taken into account, because they are very individual. The accumulation ability of hot water comprising one day has been assumed. The objective function contains the following components: the thermo-ecological cost of copper plate, copper pipes, glass plate, steel box, thermal insulation, heat transfer liquid, electricity for driving the pump of liquid, fuel for the peak boiler. The duration curves of the flux of solar radiation and absorbed heat have been elaborated according to meteorological data and used in the calculations. The objective function for economic optimization may have a similar form, only the cost values would be different.     

The optimization of solar heating systems

The optimization of a solar system with a limited number of discrete collection areas available has been discussed. This optimization is based on an exponential relationship between the auxiliary energy required and collector area. In most cases, the optimum area is that area formed by an array of collector panels which is closest to but greater than A_op. In many cases, an even larger collection area can be used without a severe economic penalty since the cost curve changes slowly as collector area is increased. Larger collector areas are also preferable from the standpoint of reducing our dependence on non-renewable fuels.     

Daylight performance of lightpipes

Increasing the use of natural daylight for lighting purposes in buildings can offer large savings in electricity usage, up to 20–30% of total building energy consumption. One solution is the use of lightpipes that can not only bring light into otherwise inaccessible or dimly lit places, but also improve the internal environment without generating excessive heat. The performance of six light pipes has been monitored in three different areas, a workshop, a residential landing, and a small office. The highest illuminance was 1538 lux, obtained underneath the straight lightpipe on the landing, with an aspect ratio of 2.1. The lowest was 41 lux in the darkest corner of the workshop. The average illuminance for the whole landing was 366 lux and a mean internal to external ratio of 0.48%. The results show that lightpipes are proficient devices for introducing daylight into buildings, the most effective lightpipes being straight, short ones with low aspect ratios; consequently, larger diameter lightpipes would probably be more effective. However, the benefits of lightpipes also include energy savings, user satisfaction and a healthier and improved indoor environment.     

Exergetic optimization of flat plate solar collectors

In this paper, an exergetic optimization of flat plate solar collectors is developed to determine the optimal performance and design parameters of these solar to thermal energy conversion systems. A detailed energy and exergy analysis is carried out for evaluating the thermal and optical performance, exergy flows and losses as well as exergetic efficiency for a typical flat plate solar collector under given operating conditions. In this analysis, the following geometric and operating parameters are considered as variables: the absorber plate area, dimensions of solar collector, pipes' diameter, mass flow rate, fluid inlet, outlet temperature, the overall loss coefficient, etc. A simulation program is developed for the thermal and exergetic calculations. The results of this computational program are in good agreement with the experimental measurements noted in the previous literature. Finally, the exergetic optimization has been carried out under given design and operating conditions and the optimum values of the mass flow rate, the absorber plate area and the maximum exergy efficiency have been found. Thus, more accurate results and beneficial applications of the exergy method in the design of solar collectors have been obtained.     

Assessment of solar emissions in the climate of Iraq for efficient multifunctional disposal

The concept of solar-hydrogen energy today provides optimal solutions in light of the climate agenda for all geographic areas of the globe.Iraq's geographical location and climate conditions predetermine ample opportunities for renewable sources, namely solar energy. High air temperatures, the prevailing number of sunny days recorded annually in the region, create a favorable technical platform for the implementation of thermal processes based on the utilization of incoming research. Therefore, for the design of solar plants and in the future, a reliable estimate of solar energy resources is required to determine the generated power depending on seasonal changes. Theoretical methods of calculating the incoming radiation flow for a particular area do not have the required accuracy for a number of reasons, the main of which is the permanent state of cloud cover. Only taking into account the actual climatic conditions for the construction areas under consideration on the basis of long-term actinometric observations gives a fairly accurate distribution of solar energy. The absence of such information in full implies forecasting of resource availability, including for areas not covered by observation systems. Given these limitations, an analysis of the average monthly daily solar radiation revenues for densely populated areas of Iraq is provided. On the basis of actinometric information of many years of observations, dependencies were obtained to determine the solar energy coming at statistical clouds per day per 1 m² of horizontal surface of the area, taking into account its latitude and serial number of the month of the year. The equations allow to fulfill the forecast of the area irradiation for the design of solar plants, to justify the subsequent efficiency of the alternative system, as well as the level of possible replacement of traditional resources and the degree of reduction of their consumption. On the basis of the calculated data, it is possible to justify the most expedient scheme of solar energy utilization depending on the available resources and to select the structures of radiation sensing devices required for climatic conditions for the solved tasks. In view of the analysis of solar energy distributions and average monthly outdoor air temperatures in Iraq, two types of thermal solar plants have been recommended for use, which may have additional circuits for generating electricity and cold. In addition, the fixed irradiation potential of the territories makes it possible to carry out an effective passive transmission to maintain the thermophilic mode of anaerobic fermentation of organic waste. In this case, environmentally friendly utilization of carbon-containing substrates is accompanied by an increase in the rate of formation of biogas, and therefore hydrogen, without the use of traditional heat supply to reactors.     

Simulation and cost optimization of solar heating of buildings in adverse solar regions

In the present study a model has been developed which simulates the effects of hourly weather conditions on the performance and cost of a combined solar/conventional heating system for buildings in cold, cloudy climates. The model exhibits the effects of several system and cost parameters on combined system cost so that optimal designs can be determined.Performance and cost results are presented for 1971 Ottawa, Ontario, weather data. The economic analysis, which treats both collector and conventional system fuel costs parametrically, shows that solar heating of a typical house in cold, cloudy climates is economically competitive with fuel oil heating only if the price of oil rises to approximately 80¢/gal.     

Load optimization in solar space heating systems

When load variables, such as window and insulation types, are included in the economic optimization of a solar space heating system, the over-all cost is lower than that resulting from optimization of collection area for a fixed load (as by FCHART [1] and SOLCOST [2]). In this paper an algorithm is derived for choosing insulation levels, as well as solar collection area, so as to minimize the over-all cost of constructing and heating a building. The general algorithm is applicable with any solar performance prediction method, and with any economic criterion where the “cost” is a linear function of collection area and of auxiliary energy consumption. A specific algorithm is also derived for active solar systems using the Relative Areas method of performance prediction [3] and a conventional present worth life cycle cost analysis. The degree-day model is used for the load calculations.     

An optimization formulation for solar heating systems

A mathematical correlation between collector area and auxiliary energy used in a solar hot water system was obtained by using TRNSYS program. Based on this correlation, optimum collector area was directly related to both economic factors and system parameters. A criteria for economic feasibility was obtained. A comparison of optimum area calculated by this analysis with optimum area based on f-chart data was in good agreement.     

Strategies in tower solar power plant optimization

A method for optimizing a central receiver solar thermal electric power plant is studied. We parametrize the plant design as a function of eleven design variables and reduce the problem of finding optimal designs to the numerical problem of finding the minimum of a function of several variables. This minimization problem is attacked with different algorithms both local and global in nature. We find that all algorithms find the same minimum of the objective function. The performance of each of the algorithms and the resulting designs are studied for two typical cases.We describe a method to evaluate the impact of design variables in the plant performance. This method will tell us what variables are key to the optimal plant design and which ones are less important. This information can be used to further improve the plant design and to accelerate the optimization procedure.     

Economic evaluation and optimization of solar heating systems

A procedure is developed for assessing the economic viability of a solar heating system in terms of the life cycle savings of a solar heating system over a conventional heating system. The life cycle savings is expressed in a generalized formby introducing two economic parameters, P₁ and P₂, which relate all life cycle cost considerations to the first year fuel cost or the initial solar system investment cost. Using the generalized life cycle savings equation, a method is developed for calculating the solar heating system design which maximizes the life cycle savings. A similar method is developed for determining the set of economic conditions at which the optimal solar heating system design is just competitive with the conventional heating system. The results of these optimization methods can be presented in tabular or graphical form. The sensitivity of the economic evaluation and optimization calculations to uncertainties in constituent thermal and economic variables is also investigated.