Computers in the design of solar energy systems

Simulations are tools of major importance in the design of solar energy systems. Only with simulations can a variety of system designs be studied and evaluated in a comparative way. Simulations are used both for systems research and as a design tool for large, one-of-a-kind systems using programs such as TRNSYS. Simulations are also the basis for development of quick design methods for standard system configurations for solar heating; these quick methods are also programmed for easy use in combination with economic analyses (e.g. f-chart).     

A general design method for closed-loop solar energy systems

A general design method is presented for closed loop energy systems consisting of solar collectors, sensible energy storage and a closed-loop flow circuit in which thermal energy is supplied (through heat exchange) to a load above a specified minimum temperature. It is assumed that the energy supplied to the load is used at a constant thermal efficiency. Computer simulations were used to estimate the long-term thermal performance of these systems, and correlations between the system performance and the system design parameters, such as the collector characteristics, load size, climatic data, and the minimum useful temperature, are presented.     

Adoption of residential solar power under uncertainty: Implications for renewable energy incentives

Many incentives at the state and federal level exist for household adoption of renewable energy like solar photovoltaic (PV) panels. Despite generous financial incentives the adoption rate is low. We use the option value framework, which takes into account the benefit of delaying investment in response to uncertainty, to examine the decision by households to invest in solar PV. Using a simulation model, we determine optimal adoption times, critical values of discounted benefits, and adoption rates over time for solar PV investments using data from Massachusetts. We find that the option value multiplier is 1.6, which implies that the discounted value of benefits from solar PV needs to exceed installation cost by 60% for investment to occur. Without any policies, median adoption time is eight years longer under the option value decision rule compared to the net present value decision rule where households equate discounted benefits to installation cost. Rebates and other financial incentives decrease adoption time, but their effect is attenuated if households apply the option value decision rule to solar PV investments. Results suggest that policies that reduce the uncertainty in returns from solar PV investments would be most effective at incentivizing adoption.     

Electrochemical systems for converting solar energy

Photoelectrochemical currents and potential distribution at the semiconductor/electrolyte interface for an electrochemical semiconductor solar energy converter have been analysed. Reactions involving minority and majority charge carriers in semiconductors and intermediate stages of electrocatalytic reactions involving electronic surface states were taken into account. The efficiency of semiconductor electrodes exposed to concentrated sunlight has been studied, and ways of increasing the photoconverter efficiency by the chemical modifications of the electrode surface have been considered for gallium arsenide and A^IIB^VI-type compounds. Characteristics of the cells of solar batteries are given.     

Criteria for justification of solar energy systems

postulate is that to be chosen as an energy source solar energy must be cheaper than alternatives. But further, where the solar energy system is a secondary power source, it must compete on the basis of the “incremental” cost of the primary energy. Reliability, environmental pollution and limitations of weight and area of the system are seen as other constraints on the choice of system. However, fuel convervation is dismissed as an irrelevant factor. The implications for developing countries are considered.     

A design procedure for solar heating systems

This paper is concerned with the design of solar space and water heating systems for residences. A simulation model capable of estimating the long-term thermal performance of solar heating systems is described. The amount of meteorological data required by the simulation in order to estimate long-term performance is investigated. The information gained from many simulations is used to develop a general design procedure for solar heating systems. The result is a simple graphical method requiring monthly average meteorological data which architects and heating engineers can use to design economical solar heating systems. A method of estimating the monthly average radiation on tilted surfaces is given in the Appendix.     

Multi-criteria evaluation for the optimal adoption of distributed residential energy systems in Japan

Evaluation of sustainable residential energy system is complex process, in which not only the economic aspect, but also the energetic and environmental effects should be taken into consideration. In this paper, an integrated design and evaluation model has been developed, by combing linear programming and multi-criteria evaluation method, in order to determine the optimal residential energy system while considering different types of information. As an illustrative example, an investigation is conducted for a typical residential building in Kitakyushu, Japan. A set of residential energy alternatives, including both conventional energy and renewable energy applications, are assumed for adoption. Based on the optimal design results from the linear programming, the various alternatives have been assessed against economic, energetic and environmental criteria. According to the evaluation results, currently, renewable energy systems are not competitive unless strong attention is paid to the environmental benefits. All electric system may be a transitional consideration before reaching an actual low carbon residential energy system. Furthermore, the evaluation result is greatly influenced by the criteria priority, as well as the evaluation method.     

Reducing institutional barriers to solar energy through the use of cooperatively-owned solar energy systems

Despite the conceptual attractiveness of solar energy systems for space-heating applications, the market penetration rate of solar systems remains low. The physical, economic, and institutional barriers to acceptance that cause this disappointing performance are closely related to the present political, social, and economic structure, and so are proving difficult and time-consuming to lower. However, aggregating small solar systems into larger cooperative ventures can avoid many of these barriers altogether. This “product-fit” approach could greatly increase the diffusion rate of solar energy systems into the economy, thereby replacing fossil-fuel uses sooner and to a greater extent than previously expected.     

Knowledge and adoption of solar home systems in rural Nicaragua

Solar home systems (SHSs) are a promising electrification option for many households in the developing world. In most countries SHSs are at an early stage of dissemination, and thus face a hurdle common to many emerging alternative energy technologies: many people do not know enough about them to decide whether to adopt one or not. This study uses survey data collected in Nicaragua to investigate characteristics that predict the knowledge and adoption of SHSs among the rural population. First, a series of probit models is used to model the determinants of four measures of SHS knowledge. Next, a biprobit model with sample selection is employed to investigate the factors that predict SHS adoption, conditional on having sufficient knowledge to make an adoption decision. Comparison of the biprobit formulation to a standard probit model of adoption affirms its value. This study identifies multiple determinants of SHS knowledge and adoption, offers several practical recommendations to project planners, and provides an analytical framework for future work in this policy-relevant area.     

Review of latent thermal energy storage systems for solar air-conditioning systems

Solar air conditioning is an important approach to satisfy the high demand for cooling given the global energy situation. The application of phase-change materials (PCMs) in a thermal storage system is a way to address temporary power problems of solar air-conditioning systems. This paper reviews the selection, strengthening, and application of PCMs and containers in latent thermal storage system for solar air-conditioning systems. The optimization of PCM container geometry is summarized and analyzed. The hybrid enhancement methods for PCMs and containers and the cost assessment of latent thermal storage system are discussed. The more effective heat transfer enhancement using PCMs was found to mainly involve micro-nano additives. Combinations of fins and nanoadditives, nanoparticles, and metal foam are the main hybrid strengthening method. However, the thermal storage effect of hybrid strengthening is not necessarily better than single strengthening. At the same time, the latent thermal storage unit has less application in the field of solar air-conditioning systems, especially regarding heat recovery, because of its cost and thermal storage time. The integration of latent thermal storage units and solar air-conditioning components, economic analysis of improvement technology, and quantitative studies on hybrid improvement are potential research directions in the future.     

Simulation of solar hydrogen energy systems

Solar hydrogen is a promising long-term global energy option for the post-fossil fuel era. On the other hand, solar hydrogen may have already found an early commercial application in the form of seasonal energy storage for remote stand-alone photovoltaic (PV) applications. In a stand-alone solar hydrogen energy system, the photovoltaic array is coupled with an electrolyser to produce H₂ which is stored to be later converted back to electricity in a fuel cell. The system setup comprises several subsystems which have to be controlled in an optimal way. Numerical simulations are used to get a closer insight into the transient response behavior of these elegant, but rather complicated systems during variable insolation conditions and to estimate the overall system performance accurately over extensive periods of time. The simulations are performed with the H2PHOTO program which has been successfully used for the design of a solar hydrogen pilot plant. It has also shown good accuracy against experimental data.     

Single- and multi-tank energy storage for solar heating systems: fundamentals

A multi-tank liquid-water system for storing low-temperature solar-derived heat is investigated experimentally and analytically. The motivation is a perceived economic advantage of the proposed system over single-tank systems in solar heating systems where the required total volume of water is rather large — say 2000 l or more. In the proposed system, the individual tanks in the multi-tank system (each with a volume of about 200 l) are interconnected by means of two strings of immersed-coil heat exchangers: the first string serially connects exchangers that have been immersed at the bottoms of the tanks, the second connects exchangers that have been immersed at the tops of the tanks. The first string will be connected to the solar collector loop and the second in the load loop. The present work experimentally demonstrates the degree of effective stratification that the multi-tank system can achieve, as well as a thermodynamically advantageous ‘thermal diode’ effect. It also describes a model for a multi-tank system as well as experiments that validate the model. Part of the multi-tank model is a model for a single tank with immersed coil heat exchanger, and this model drew upon a ‘reversion-elimination’ algorithm from the recent literature. The validity of the reversion-elimination algorithm is supported by the present experiments.     

A simplified method for optimal design of solar water heating systems based on life-cycle energy analysis

Significant energy mismatch exists in solar water heating systems as the time and amount of solar energy supply are usually different from that of hot water demand. Using a hot water storage tank can reduce or eliminate such mismatch in short term while it is difficult to avoid this mismatch in long term. In many optimal design and life-cycle analysis methods, the energy mismatch is ignored which causes the system performance to be overestimated and also misleads the optimal design of the system. This paper presents a simplified method for optimizing the key parameters of solar water heating systems based on life-cycle energy analysis. This optimal method considering the energy mismatch phenomenon can be implemented through two steps. In the first step, a simplified energy model based hourly energy matching different components of the system, is developed for determining the operating performance of system with different solar collector areas and water storage volumes. In the second step, the law of diminishing marginal utility is employed to determine the optimum size of the system. The optimum size is identified when the maximal life-cycle net energy saving is achieved. A case study on the application of the proposed method in a building is presented as well.     

Low-profile heliostat design for solar central receiver systems

Heliostat designs intended to reduce costs and the effect of adverse wind loads on the devices were developed. Included was the low-profile heliostat consisting of a stiff frame with sectional focusing reflectors coupled together to turn as a unit. The entire frame is arranged to turn angularly about a center point. The ability of the heliostat to rotate about both the vertical and horizontal axes permits a central computer control system to continuously aim the sun's reflection onto a selected target. A schematic of the heliostat design is shown in Fig. 1. An engineering model of the basic device was built and is being tested. Control and mirror parameters, such as roughness and need for fine aiming, are being studied. The fabrication of these prototypes is in process. The model was also designed to test mirror focusing techniques, heliostat geometry, mechanical functioning, and tracking control. The model can be easily relocated to test mirror imaging on a tower from various directions. In addition to steering and aiming studies, the tests include the effects of temperature changes, wind gusting and weathering. The results of economic studies on this heliostat are also presented.     

Optimum design criteria for solar hot water systems

This paper is concerned with the optimisation of some design criteria of SHW systems intended for residential and hotel applications. For this purpose, a system model based on TRNSYS programme has been used to correlate the performance and cost effectiveness of the system with a number of key design criteria which include the Collector to Consumer Factor (FCC) expressed in m² of collector per consumer and the Collector to Load Factor (FCL) expressed in m² of collector per annual GJ of thermal load.     

A design procedure for solar air heating systems

A natural extension of the design procedure for liquid-based solar space and water heating systems is a similar analysis for solar heating systems using air as the heat transfer fluid. In this paper, a solar air heating system incorporating a flat-plate air heater and packed bed thermal storage is described and a simulation model for the system is developed. The results of many simulations of the air heating system are used to establish the relationship between system performance and the system design and meteorological variables. The results are presented in analytic and graphical form, referred to as an f-chart for solar air heating systems. The results of simulations in several widely different climates suggest that the information presented in the f-chart is location independent. Methods of estimating the performance of air heating systems having a collector air capacitance rate and a storage capacity other than those used to generate the f-chart are included. A comparison of the performance of air and liquid based systems is afforded by a comparison of their respective f-charts. The air system is shown to perform better at high load fractions supplied by solar energy than a liquid-based system with the same collector thermal performance parameters.     

Multicriteria approach for the improvement of energy systems design

An iterative procedure is suggested to evaluate and improve the energy system design. The procedure considers the information deriving from complementary evaluation approaches, each applied within its appropriate time-space window of interest: (a) Process-related, local-scale methods (Energy, Exergy and Thermoeconomic analyses); (b) Environmental assessment methods (Impact Assessment, Emergy Synthesis); (c) Economic methods (Micro- and Macro-Economic and Externality Evaluations). Process-related methods are applied first, in order to provide local-scale performance indicators able to suggest optimization procedures from a user-side point of view. Environmental evaluation approaches are then used to judge the overall environmental quality of the design, in the largest regional and biosphere scales. Finally, micro- and macro-economic evaluation approaches are applied in order to ascertain the soundness of the proposed solution as far as the economic return on the investment as well as global benefits to the Society are concerned. New choices for the design configuration and parameters may be suggested by implementing the iterative procedure. A cogeneration system working in a town of Northern Italy is used as a case study: starting from the present configuration of the plant, modifications are suggested and evaluated, thus identifying the way for improving the performance under various viewpoints. A proposal for plant transformation from Steam Cycle to Combined Cycle is suggested, capable of increasing the plant electric power from 136 to 332 MWe, increasing the energy efficiency from 60 to 63%, increasing the exergy efficiency from 37 to 49%, and decreasing the overall demand for environmental support (transformity) from 1.84×10⁵ to 1.27×10⁵ seJ/J. The paper points out several benefits and bottlenecks of the existing plant and of the proposed solutions.