Performance Of Solar Thermal Systems Research Articles

Development of an analytical model for the daily performance of solar thermal systems with experimental validation

Flat plate collectors are the most widespread solar thermal collectors for low-temperature level applications as domestic hot water production and space heating. The objective of this study is to present a simple and analytical model for the prediction of the daily performance of an integrated system with flat plate collector. The developed model is tested with experimental results from a laboratory integrated system. This experimental set-up is investigated with details in order to determine its thermal performance and its dynamic response. After the validation of the developed model, the model is extended to realistic weather data for the climate conditions of Athens (Greece). The integrated system is evaluated energetically and financially by examining twelve typical days, one for every month. It is found that the investigated system can produce 2171 kW h thermal energy during the year with a mean thermal efficiency equal to 54.24%. The payback period is calculated at 5.03 years, an acceptable value which indicates sustainability. The present analysis clearly indicates that the developed model can be used for quick and accurate calculations on solar thermal systems in order to evaluate and optimize them on a daily basis.

Effect of phase transition temperature and thermal conductivity on the performance of Latent Heat Storage System

The heat transfer properties of phase change materials (PCMs) are of importance for the efficiency assessment on the heat storage and release in solar thermal systems. Previous research results demonstrate that the increase of thermal conductivity of PCMs can enhance the thermal performance in solar thermal systems; however, the corresponding mechanism is not clear. To this end, this work investigates the influence of PCMs properties on storage performance of solar thermal systems. First, experimental testing was conducted to verify the effectiveness of a thermal simulation model in the heat storage and release process. Then, the proposed simulation model was used to investigate the performance of several commonly used PCMs in the process of melting and solidification. The influence of thermal conductivity and phase transition temperature on the thermal storage properties was analyzed. The analysis results demonstrated that the influence of phase transition temperature on the thermal system performance was greater than that of the thermal conductivity in short time, while the thermal conductivity contributed greater influence on the system performance in long time. The phase transition temperature hardly affected the total system efficiency if given enough heat transfer time. The findings in this work may provide a theoretical reference for the selection of heat storage materials.

Performance comparison of latent heat storage systems comprising plate fins with different shell and tube configurations

For design and optimization purposes, understanding the performance characteristics of different configurations of PCM systems during charging and discharging processes is crucial, particularly for thermal storage in CSP. A numerical study using FLUENT has been performed to explore and compare the characteristics and behavior of PCM in different shell and tube configurations; namely counter flow and parallel flow in vertical and horizontal orientations. These designs have been compared with that of an alternative design using plate fins. The main practical advantage of the alternative design is decoupling the PCM location from the path of flow of the heat transfer fluid that reduces the constraints on the materials. Confined heating/cooling pipes in a channel at the bottom of the PCM enclosure makes it more flexible to match the required working pressures, and simplifies the design and material (PCM/plate) selection. This is a significant issue as higher temperatures and pressures are being required for improving the overall solar thermal system performance. Based on the same heat transfer area, amount of PCM and input energy, a performance comparison was determined based on exergy maximization. Using sodium nitrate as the PCM, the results show a higher heat transfer rate for the vertical arrangement of plate fins compared to the counter flow shell and tube configurations, as well as a more uniform heat transfer rate compared to all shell and tube arrangements for both the charging and discharging processes. The proposed design leads to less redundant PCM, as well as a smaller and more cost effective PCM system as a heat storage unit. Overall, the vertical plate fin configuration maximizes the useful heat that can be extracted for a given amount of heat and PCM, which has direct applicability to thermal storage solutions for CSP and other applications. Moreover, its modular design facilitates the selection and optimisation for different scales and applications e.g. as a heat storage for electricity production or as a protection system for a receiver from high thermal stresses in a CSP plant.

CFD modelling development and experimental validation of a phase change material (PCM) heat exchanger with spiral-wired tubes

Employing phase change materials (PCMs) for latent heat storage (LHS) application has a great potential to improve a solar thermal system performance. Despite this fact, the use of PCM in this area is quite limited due to the poor thermal conductivity of available PCMs. Therefore, heat transfer enhancement is one of the essential strategies that can overcome this obstacle. In this paper and related project, a PCM heat exchanger (HX) is purposely designed with spiral-wired tubes and integrated in an indirect solar assisted heat pump test system. Although the spiral-wired tube has not been applied in a PCM HX, it is expected to enhance significantly the PCM heat transfer and heat storage performance. To verify this and understand the PCM heat storage and releasement processes, a detailed 3D CFD model has been developed for the PCM HX and validated with measurements. The temperature variations and visualizations of the PCM during charging and discharging processes are therefore simulated and presented temporally. Furthermore, the effects of different inlet heat transfer fluid flow rates and temperatures on the PCM melting/solidification time are demonstrated in this study. Some significant simulation results have been obtained which can instruct efficiently the operation of the heat exchanger and its integration with the solar system.

Transient Uncertainty Analysis in Solar Thermal System Modeling

Complex, dynamic, computational models are routinely used to evaluate and optimize the design and performance of solar thermal systems. As models become more complex, performing uncertainty analysis on such models can be quite challenging and computationally expensive. This paper presents an effective approach to quantify uncertainties associated with transient simulation results from a dynamic solar thermal energy system model with uncertain parameters. The proposed method utilizes the concept of impulse response and convolution process to estimate the sensitivities to time-varying external inputs. Using this method, the number of simulations required to propagate uncertainties through dynamic models can be significantly reduced. An example is presented throughout the paper to demonstrate the procedure of the proposed uncertainty analysis approach.

Energy and Environmental Performance of Solar Thermal Systems in Hotel Buildings

The energy policies in Europe include the reduction in energy consumption of buildings along with their environmental impact, focusing mainly on CO2 emissions. In this framework, solar thermal systems used for space heating purposes and Domestic Hot Water production will be studied in detail. Solar thermal systems are one of the most popular ways to provide sanitary hot water in hotel buildings, all over the world and in particular in the Mediterranean. This is due to the fact that over the last forty years they have been proven to be reliable and cost effective.Still, there is more to those systems, than cost efficiency, as it will be discussed in this paper, based on a study carried out for the Greek hotel sector. The analysis of the solar thermal system was done on the basis of technical characteristics and their energy efficiency as well as of the potential energy saving and their environmental impact, focusing mainly on CO2 emissions. The environmental evaluation of the system studied is expressed by means of an integrated environmental approach, based on environmental rating systems namely LEED and BREEAM.Goal of the paper is to present the result of the overall evaluation of the solar thermal system and its contribution to the improvement of the building's energy and environmental performance and also discuss how this affects the building's respective certification.

Analysis of solar thermal systems and future development possibilities in Lithuania

Solar thermal systems with a total solar panel area varying from 2 to 204 m2 have been installed in Lithuania for over 20 years. The reviewed solar thermal domestic hot water systems in Lithuania produce up to 528 kWh per year per one square meter of solar collector absorber area. However, the performance of these systems varies depending on the type of energy users, equipment and design of the systems, as well as their maintenance. The aim of this paper was to analyse solar thermal systems from the perspective of energy production and economic benefit as well as to outline the differences of their actual performance compared to the numerical simulation results. A number of different solar thermal systems in Lithuania were selected for the analysis varying both in equipment used (flat type solar collectors, evacuated tube collectors) and type of energy user (domestic hot water heating, swimming pool, district heating). The simulation software Polysun 8.1 was used for evaluation of solar thermal system performance, as well as financial analysis of alternatives of these systems. The results of the analysis showed that in the analysed cases the gap between measured and modelled data of heat energy produced by solar thermal systems was up to approximately 11%. The calculation of internal rate of return showed that a grant is required in most cases for solar thermal projects to be fully profitable.

Exergy based performance evaluation of solar air heater with arc-shaped wire roughened absorber plate

Exergy efficiency analysis is an useful method to evaluate overall performance of solar thermal systems as it takes into consideration of useful energy output and consequent pumping power requirement. In present work, an investigation on exergetic performance evaluation of solar air heater with arc-shaped wire rib roughened absorber plates has been made analytically by employing mathematical model and the results have been compared with a plane absorber plate solar air heater for similar operating conditions. The exergetic efficiency curves as a function of Reynolds number (Re) and temperature rise parameter (ΔT/I) for different roughness parameters have been plotted. The maximum enhancement in exergetic efficiency of roughened solar air heater as compared to smooth absorber plate solar air heater has been found as 56% corresponding to relative roughness height (e/D) = 0.0422. The design plots, exhibiting the optimum combination of roughness parameters, can be used to design arc shaped wire rib roughened solar air heater.

家庭用太陽熱給湯器のエネルギー収支とライフサイクルCO<sub>2</sub>排出量

Objective. The objective of the present study is to evaluate the environmental performance of solar thermal systems for domestic hot-water supply from life cycle perspective. Energy ratios and life cycle CO2 emissions of two different types of solar thermal system are analyzed, namely, the integrated solar thermal system and the separated solar thermal system that supply 40% of the total hot-water demand of a four people household. This analysis uses the material and energy data collected from eight manufacturers of solar thermal systems to reflect the present reality. Results and Discussion. The analysis shows that the life cycle CO2 emissions of solar thermal systems are 25-37% less than those of domestic gas boilers. Energy ratios of solar thermal systems are more than one, while that of a gas boiler system is 0.9. This is because solar thermal systems require less gas consumption compared to a domestic gas boiler. Moreover, comparison between the two different types of solar thermal systems reveals that the life cycle CO2 emissions and energy consumption of the separated solar thermal system are more than those of the integrated solar thermal system mainly because the separated type consumes electricity for pumps to circulate the heat medium. Furthermore, The effects on CO2 emission of the change in heat supply rate by the solar thermal systems are analyzed. The analysis finds that the more the heat supply rate increases, the less the CO2 emissions of the solar systems are. When the solar thermal systems supply 60% of the total hot-water demand, the life cycle CO2 emissions of solar thermal systems are 25-37% less than in the case of 40% supply. Conclusions. The present study reveals that both the integrated and separated solar thermal system are superior to gas boiler from viewpoints of energy ratios and life cycle CO2 emissions. Furthermore, comparison between the integrated and separated solar thermal system finds the former has better environmental performance than the latter.

Year-round performance assessment of a solar parabolic trough collector under climatic condition of Bhiwani, India: A case study

Solar parabolic trough collector (SPTC) is a well-known solar thermal system applied for solar electric generation. Nowadays, major attention is directed toward improving the performance of solar thermal systems with optimization of solar field production. In this research work, a comprehensive thermo-optical modeling has been proposed to evaluate the performance of a mini-level SPTC considering various heat equilibriums with the environment. Here, receiver wall temperature is considered as the base for modeling. Collector consists of a non-evacuated receiver tube with black paint coating and enveloped with glass cover. Available meteorological data in terms of global and diffuse solar insolations, air temperatures and wind speeds have been used as inputs for performance evaluation of SPTC with horizontal and inclined aperture planes. The validation of the proposed analytical model is justified with existing experimental results and yielded a close agreement. The developed model is successfully applied to a SPTC in order to estimate the through-out year performance characteristics in terms of water temperature rise, heat energy generation, optical and thermal efficiency for the climactic conditions of Bhiwani. The results enlighten that using 0.010kg/s mass flow rate of water and aperture area of around 1.34m2, collector achieved maximum rise in water temperature 11.1°C and 12.2°C on horizontal and inclined planes, respectively in the month of April. The uppermost heat energy generation is found to be 2.38kWh/day in May on horizontal plane, 2.46kWh/day in April on inclined plane. The maximum optical efficiency is attained as 72.26% and 72.4% on horizontal and an inclined plane respectively. The peak instantaneous thermal efficiency is accomplished as 66.78% in July on horizontal plane, 65.77% in September on inclined plane of collector.

Analysis of entropy generation in double-diffusive natural convection of nanofluid

In the present work, entropy generation analysis on double-diffusive natural convection of nanofluid in a rectangular enclosure is conducted to deepen our insights into the performance of solar thermal systems. The effects of thermal Rayleigh number, ratio of buoyancy forces, nanoparticle volume fraction and aspect ratio of enclosure are discussed in details. It is found that entropy generation will be enhanced more intensively in turbulent regimes. The total entropy generation will reach its minimum when the ratio of buoyancy forces is unity. The total entropy generation is a monotonic decreasing function of nanoparticle volume fraction. It is interesting that the irreversibility due to thermal diffusion, viscous dissipation and concentration diffusion will change about the same rate with nanoparticle volume fraction. The differences and similarities between nanofluid and pure base fluid are also analyzed. In some scenarios, the data based on the second law of thermodynamics are better than that based on the first law of thermodynamics to describe the difference between heat and mass transfer intensity. Moreover, we find a kind of self-organized phenomenon, which we firstly observed in double-diffusive natural convection of air, also exists in its nanofluid counterpart. Especially, even in fully turbulent regimes, there appear order structures in the picture of relative entropy generation. Such phenomenon can be used as an indicator to find order in complicated flow.

Thermal Reliability of Paraffin Wax Phase Change Material for Thermal Energy Storage

Phase change materials (PCMs) as thermal energy storage medium are proven to be effective to enhance the performance of solar thermal system. The degradation of the thermal properties due to thermal cycles is changeable and accordingly the performance of the solar thermal cycle may decline. In this study, the thermal reliability of paraffin wax was investigated to analyse the ability to be used as thermal energy storage (TES) for solar water heating purposes that subjected to many phase change cycles. The mixtures were subjected to 400 phase change cycles and the thermal properties were measured. Two samples were prepared; Sample 1 was paraffin wax without phase change cycles whereas Sample 2 was gone through 400 phase change cycles. Four hundred phase change cycles indicated the phase change cycles for 1 year 35 days as 1 cycle equivalent to 1 day. The comparison of samples with and without 400 phase change cycles showed slight changes in thermal conductivity, specific heat, melting point and solidification point. Fourier Transform Infrared Spectrometer and Thermogravimetric Analysis showed that after 400 phase change cycles there is no weight loss observed. The paraffin wax is hence found reliable to be use without any degradation, without any chemical reaction and slightly improvement of thermophysical properties as TES for solar water heating purposes.

Representative hot water draw profiles at high temporal resolution for simulating the performance of solar thermal systems

The heating of domestic hot water (DHW) to supply bathing, cleaning, and appliance needs contributes significantly to energy demands in housing. Despite the demonstrated significance of DHW draw profiles on performance, it is common practice when analyzing solar DHW and solar combisystems to conduct long-term simulations (e.g. annual) using repeating daily profiles. New DHW profiles that can be used for such analyses have been developed based upon measurements that were taken in 73 houses in Québec (Canada). Annual DHW draw profiles at a time-resolution of 5min have been created for 4 consumption levels and 3 temporal consumption patterns (sparing consumers who tend to use DHW in the mornings, average consumers who tend to use DHW in the evenings, etc.). Simulations of a solar DHW system conducted with these new profiles revealed that both temporal and annually integrated predictions can be significantly affected by timing of DHW draws.

Effective solar-thermal collector with uniform concentration

Solar concentrators are essential to reach high temperatures in solar-thermal applications. The high temperature is required to achieve effective heat transfer between different sections in solar-thermal systems that are utilized to produce electricity from the produced steam. The commonly used parabolic trough and cylindrical reflectors do not provide a uniform concentration on the receiving tube; therefore, in this paper a reflector profile design that uniformly concentrate the sunlight onto the receiver was attempted and was accomplished by solving a second order differential equation numerically. Unlike the parabolic reflector, the proposed reflector was found to exhibit more uniform concentration profile; therefore it is expected to enhance the conversion efficiency and the overall solar-thermal system performance.

Modelling the effect of realistic domestic energy demand profiles and internal gains on the predicted performance of solar thermal systems

This work investigates the importance and consequent complexity of gaining a reliable estimate of the temporal energy demands made of active domestic solar systems. Twelve typical Welsh dwellings are modelled in TRNSYS. The influence of weather data, thermal comfort operating schedule, lighting and plug loads, on the predicted thermal energy demands that are to be met by solar thermal combi-systems with heat storage is studied. It is revealed that the balance between potential solar contribution and the need for heat storage are strongly influenced by these input variables, and the effects are more marked in recent, heavily insulated housing. Consideration of the potential range of demands for space heating and cooling of a particular house type is thus found to be imperative in designing a system that is efficient and robust across varying occupancy and climatic conditions. Furthermore the temporal relationship of energy supply and demand profiles could be the main determinant of the appropriate strategy for meeting the energy needs and system aims efficiently. The study demonstrates also that dynamic system simulation tools like TRNSYS can handle the complexity of elaborate building modelling descriptions but highlights the need for more suitable modelling methods which incorporate comprehensive, building-focused interfaces.

Uncertainty in estimating the performance of solar thermal systems

The scope of this article is to present a methodology for the estimation of the uncertainty characterizing the energy performance of solar domestic hot water systems. The work concentrates on the uncertainty characterizing the expected annual energy output, as calculated through tests implemented according to the valid international standards. In order to cope with difficulties related to the algorithmic character of the measurement model, which cannot be explicitly formulated, Monte-Carlo simulation techniques are implemented. The component of uncertainty associated with measurement errors is estimated, on the basis of the metrological quality anticipated by the relevant testing standards. Errors due to imperfections of the energy model used through the test are also counted in, as well as uncertainties attributed to the variability of meteorological conditions. The proposed uncertainty analysis allows the realistic assessment of the actual energy provided to the user by a solar domestic hot water system. Implementation of the proposed methodology for a typical system leads to an expanded uncertainty in the order of 9% for the expected annual energy output.

Nomograph for rapid technical and economic assessment of solar thermal systems for DHW production

Different types of tools, software and empirical methods to size and assess the performance of solar thermal systems are available today. A quick and easy-to-use graphical tool is proposed here. It is based on a single diagram, called a nomograph, which embeds technical and economic information, allowing the optimisation of the size and performance of a solar thermal system starting from the main input parameters, such as the specific costs of the system and the auxiliary fuel. The optimal surfaces are calculated as a function of the fuel cost through several regressions made on data from the application of the F-Chart method for a set of system configurations. The authors have found good correlations among these two parameters for given collectors types, climates and fuel costs.Furthermore, the design information stemming from the use of the diagram considers the optimal area of the surface of solar collectors, the corresponding annual solar fraction, the annualised total cost of the solar system and the conventional systems meeting the same load, the annual savings achieved with the use of solar system and the corresponding payback time. Nomographs have been drawn for three Italian locations, representative of different South Europe climates, for a given demand of hot water and for two types of collectors. The methods and algorithms related to the construction of the diagrams are described in this paper. Additional nomographs can be produced through a standardised procedure based on Excel Macros. An analysis of the results obtained is then performed for a sample application.

The Influence of Energy Conservation on the Performance of Solar Thermal Systems – A Cold Country Case Study

The European Union has set a goal that the energy use in the built environment shall be reduced by 41% to the year 2050 compared to 2005-2006. This could introduce new opportunities for solar thermal systems in cold countries. In such countries, like Sweden and Canada, the economy in solar thermal collector installation projects is often spoiled by the fact that most of heating energy demand of the building occurs during periods when the available solar energy is low.The present paper investigates the performance of solar thermal systems subjected to different quota between space heating and domestic hot water demand (DHW). This study investigates the performance of a solar thermal system integrated to four different buildings with varying heating loads in two different locations, Sweden and Canada. Models of single family houses are created which are able to simulate the total heating demand with different heating demand profiles but the same DHW demand. Simulations are performed in TRNSYS, an advanced tool used to simulate transient systems. Results indicate that solar combisystems tend to generate more useful energy and therefore be more cost effective when installed in buildings with higher heating demands.

Use of a Shroud and Baffle to Improve Natural Convection to Immersed Heat Exchangers

Optimizing heat transfer during the charge and discharge of thermal stores is crucial for high performance of solar thermal systems for domestic and commercial applications. This study models a sensible water storage tank for which discharge is accomplished using a heat exchanger immersed in the storage fluid. The heat exchanger is a two-dimensional isothermal cylinder in an adiabatic enclosure with no initial stratification. An adiabatic shroud and baffle whose geometry is parametrically varied is placed around and below the cylinder. Transient numerical simulations of the discharge process are obtained for 105 < RaD < 107, and estimates of the time needed to discharge a given fraction of the initial stored energy are obtained. We find that a short baffle is least effective in increasing heat transfer rates. The performance benefit is greatest early in the transient discharge period when the buoyant flow in the store is strongest. As with all flow control devices, the benefit decreases as energy is extracted from the tank and the temperature difference driving the flow decreases. The use of a shroud increases the transient Nusselt number by as much as twentyfold.

The real utility ranges of the solar selective coatings

The efficiency of the solar selective coatings ( η sel) with various combinations of the optical properties (solar absorptance and thermal emittance), and their impact on the performance of solar thermal systems of different concentration ratios (CRs) have been analyzed. The stagnation temperatures of the selective coatings have been measured using stagnation temperature measurement (STM) chamber. From the results of the simulation study, it is recommended that selective solar absorber coatings should be used only in systems with CR=1 (solar flat plate collectors) and its use in systems with high CR values (parabolic collector) is an additional expenditure with reduced efficiency. The dependence of the performance of the system with CR=1 on the low emittance of the absorber coating has also been experimentally confirmed by the stagnation temperatures recorded for different selective coatings measured in an STM chamber, a system with unit CR. For systems with CR=1, a simple and new parameter, specific area ratio (SAR) has been proposed which further pinpoints the exact solar system wherein the selective coatings are to be used.