Simulation and experimental investigation of two hybrid solar domestic water heaters with drain water heat recovery

In this paper, the performance of two solar domestic hot waters (SDHW) with drain water heat recovery (DWHR) units is investigated. Both SDHW systems are recently installed at the Archetype Sustainable Twin Houses at Kortright Center, Vaughan, Ontario. The first SDWH system in House A consists of a flat plate solar thermal collector in combination with a gas boiler and a DWHR unit. The second SDHW system in House B includes an evacuated tube solar collector, an electric tank, and a DWHR unit. Both systems are modeled in TRNSYS, and the models are validated by experimental data. The addition of the DWHR and the flat‐plate solar thermal collector would result in 1831 kWh of annual energy saving in House A. While the addition of the DWHR and the evacuated tube collector in House B would result in an annual energy saving of 1771 kWh. Subsequently, the models are used to investigate the performance of similar systems for five major Canadian cities of Halifax, Montreal, Toronto, Edmonton, and Vancouver. The conjunctions of solar thermal collectors with DWHR units are found most beneficial in Edmonton. It is also noted from experimental and simulated results that flat‐plate solar collector‐based water heater produced more thermal energy than the system based on the evacuated tube solar collector for all major Canadian cities. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical variations of the thermal performance of different solar collectors and solar combi systems as function of the varying yearly weather conditions in Denmark

The thermal performances of solar collectors and solar combi systems with different solar fractions are studied under the influence of the Danish design reference year, DRY data file, and measured weather data from a solar radiation measurement station situated at the Technical University of Denmark in Kgs. Lyngby. The data from DRY data file are used for any location in Denmark. The thermal performances of the solar heating systems are calculated by means of validated computer models. The measured yearly solar radiation varies by approximately 23% in the period from 1990 until 2002, and the investigations show that it is not possible to predict the yearly solar radiation on a tilted surface based on the yearly global radiation.The annual thermal performance of solar combi systems cannot with reasonable approximation be fitted to a linear function of the annual total radiation on the solar collector or the annual global radiation. Solar combi systems with high efficient solar collectors are more influenced by weather variations from one year to another than systems with low efficient solar collectors.The annual thermal performance of solar collectors cannot be predicted from the global radiation, but both the annual thermal performance and the annual utilized solar energy can with a reasonable approximation be fitted to a linear function of the yearly solar radiation on the collector for both flat plate and evacuated tubular solar collectors. Also evacuated tubular solar collectors utilize less sunny years with large parts of diffuse radiation relatively better than flat plate collectors.     

Energy and exergy analysis of hybrid photovoltaic thermal water collector for constant collection temperature mode

This paper deals with the analysis of hybrid photovoltaic thermal (PVT) water collectors under constant collection temperature mode unlike constant flow rate mode. The analysis has been carried out in terms of thermal energy, electrical energy and exergy gain for two different configurations namely case A (collector partially covered by PV module) and case B (collector fully covered by PV module). The results are compared with the conventional flat plate collector (FPC). The effect of collector area covered by PV module on the performance of hybrid PVT water collector has been studied. The characteristic equations have also been developed for both the cases.It has been observed that case A is more favorable for thermal energy point of view, while case B is suitable for electricity generation. On the basis of the numerical calculations the annual thermal energy gain is found to be 4167.3 and 1023.7 and annual net electrical energy gain is 320.65 and 1377.63 for cases A and B respectively. The annual overall thermal energy gain is decreased by 9.48% and an annual overall exergy gain is increased by 39.16% from case A to case B.     

Experimental investigation on the thermal efficiency and performance characteristics of a flat plate solar collector using SiO2/EG–water nanofluids

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO₂/ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO₂ nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles.     

A comparison between state-of-the-art and neural network modelling of solar collectors

The state-of-the-art modelling of solar collectors as described in the European Standard EN 12975-2 is based on equations describing the thermal behaviour of the collectors by characterising the physical phenomena, e.g. transmission of irradiance through transparent covers, absorption of irradiance by the absorber, temperature dependent heat losses and others. This approach leads to so called collector parameters that describe these phenomena, e.g. the zero-loss collector efficiency η₀ or the heat loss coefficients a₁ and a₂.Although the state-of-the-art approach in collector modelling and testing fits most of the collector types very well there are some collector designs (e.g. “Sydney” tubes using heat pipes and “water-in-glass” collectors) which cannot be modelled with the same accuracy than conventional collectors like flat plate or standard evacuated tubular collectors. The artificial neural network (ANN) approach could be an appropriate alternative to overcome this drawback.To compare the different approaches of modelling investigations for a conventional flat plate collector and an evacuated “Sydney” tubular collector have been carried out based on performance measurements according to the European Standard EN 12975-2. The investigations include the parameter identification (training), the comparisons between measured and modelled collector output and the simulated yearly collector yield for a solar domestic hot water system for both models.The obtained results show better agreement between measured and calculated collector output for the artificial neural network approach compared with the state-of-the-art modelling. The investigations also show that for the ANN approach special test sequences have to be designed and that the determination of the ANN that fits the thermal performance of the collector in the best way depends significantly on the expertise of the user.Nevertheless artificial neural networks have the potential to become an interesting alternative to the state-of-the-art collector models used today.     

Correlations for natural convective heat exchange in CPC solar collector cavities determined from experimental measurements

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, C_R = 2 and a 100 mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.     

Longevity characteristics of flat solar water-heating collectors in hot-water-supply systems. Part 1. Procedure for calculating collector thermal output

A procedure for calculating longevity indices (daily and monthly variations and, hence, annual thermal output) of flat solar water-heating collectors, amount of conditional fuel saved per year by using solar energy, and cost of solar fuel and thermal energy generated in hot-water-supply systems is described.     

The experimental study of energy features for solar air heaters with different turbulator configurations

To assess the thermal performance in the climate conditions of western and central Iraq, the advantages of using a solar air collector with various turbulator absorber plates are experimentally explored. Four distinct kinds of absorber plates are provided flat plate (F), triangular (T), rectangular (R), and circular (C) turbulators at different air mass flow rates. The collector's economic properties and overall thermal performance are compared to the conventional flat plate turbulator heating systems. The main findings suggest that delta turbulators improve collector economics and overall thermal performance by generating vortex and dampening the formation of the thermal boundary layer in the direction of airflow. Furthermore, when the mass flow rate increases, the thermal performance improves, and the efficiency increases for all mass flow rates, resulting in good thermal performance for the rectangular plate collector when compared to other collectors. When compared to other types of configurations, the daily average efficiency of solar air collectors for flat plate (F), triangular (T), rectangular (R), and circular (C) turbulators are 28%, 67%, 39%, and 48%, respectively, at 50° tilt angle while at 90° tilt angle they are 44%, 76%, 54%, and 63%, respectively, as ṁ = 0.0377 kg/s. The maximum daily average efficiency fitted with rectangular turbulators have about 86% at the largest ṁ = 0.1 kg/s. This study will also give a unique direction to the work trend in the western and central parts of Iraq throughout the winter months.     

Simple procedure for predicting long term average performance of nonconcentrating and of concentrating solar collectors

The Liu and Jordan method of calculating long term average energy collection of flat plate collectors is simplified (by about a factor of 4), improved, and generalized to all collectors, concentrating and nonconcentrating. The only meteorological input needed are the long term average daily total hemispherical isolation on a horizontal surface and, for thermal collectors the average ambient temperature. The collector is characterized by optical efficiency, heat loss (or U-value), heat extraction efficiency, concentration ratio and tracking mode. An average operating temperature is assumed. If the operating temperature is not known explicitly, the model will give adequate results when combined with the , f-chart of Klein and Beckman.A conversion factor is presented which multiplies the daily total horizontal insolation to yield the long term average useful energy delivered by the collector. This factor depends on a large number of variables such as collector temperature, optical efficiency, tracking mode, concentration, latitude, clearness index, diffuse insolation etc., but it can be broken up into several component factors each of which depends only on two or three variables and can be presented in convenient graphical on analytical form. In general, the seasonal variability of the weather will necessitate a separate calculation for each month of the year; however, one calculation for the central day of each month will be adequate. The method is simple enough for hand calculation.Formulas and examples are presented for five collector types: flat plate, compound parabolic concentrator, concentrator with east-west tracking axis, concentrator with polar tracking axis, and concentrator with 2-axis tracking. The examples show that even for relatively low temperature applications and cloudy climates (50°C in New York in February), concentrating collectors can outperform the flat plate.The method has been validated against hourly weather data (with measurements of hemispherical and beam insolation), and has been found to have an average accuracy better than 3 per cent for the long term average radiation available to solar collectors. For the heat delivery of thermal collectors the average error has been 5 per cent. The excellent suitability of this method for comparison studies is illustrated by comparing in a location independent manner the radiation availability for several collector types or operating conditions: 2-axis tracking versus one axis tracking; polar tracking axis versus east-west tracking axis; fixed versus tracking flat plate; effect of ground reflectance; and acceptance for diffuse radiation as function of concentration ratio.     

Comparative thermal performance evaluation of an active solar distillation system

In this paper, thermal models of all types of solar collector‐integrated active solar stills are developed based on basic energy balance equations in terms of inner and outer glass temperatures. In this paper, hourly yield, hourly exergy efficiency, and hourly overall thermal efficiency of active solar stills are evaluated for 0.05 m water depth. All numerical computations had been performed for a typical day in the month of 07 December 2005 for the climatic conditions of New Delhi (28°35′N, 77°12′E, 216 m above MSL). The thermal model of flat‐plate collector integrated with active solar still was validated using the experimental test set‐up results. Total daily yield from active solar still integrated with evacuated tube collector with heat pipe is 4.24 kg m^?2 day^?1, maximum among all other types of active solar stills. Copyright © 2007 John Wiley & Sons, Ltd.     

Economic appraisal of small-scale solar organic Rankine cycle operating under different electric tariffs and geographical conditions

In the present paper, the economic feasibility of small-scale solar organic Rankine cycle power applications which are assisted with auxiliary gas heaters is investigated. The system is analyzed using three different capacities of ORC system with R-245fa (35, 65, and 110 kWe) in combination with solar water heating system (SWHS) using three models. Flat plate, compound parabolic and evacuated tube solar collectors were used to generate heat with overall heat transfer coefficient (F_RU_L) of 4.35, 1.57, and 2.23 W/m². K respectively. System Advisor Model (SAM) is used to simulate the solar water heater system and optimize the tilt angle, collector area, volume of storage tank and capacity of auxiliary heater under the climatic conditions of Abu Dhabi, New Delhi, Larnaca, Madrid and Munich. The simulation results revealed that the evacuated tube and the compound parabolic collectors performed better than the flat plate collectors. The economic analysis showed that Solar ORC Power Plant is economically and technically feasible with all types of the thermal collectors in Famagusta/Larnaca, Munich and Madrid where the electricity tariff is higher than other cities. Levelized cost of energy (LCOE) is calculated using mathematical model and it ranges between 0.07 and 0.2 USD/kWh based on the plant capacity and type of thermal collectors. Moreover, the profit increase as the plant capacity increase where SIR is 1.05, 1.71, and 2.10 for 35, 65, and 110 kW plant capacity SORC with CPC. A sensitivity analysis is also performed to investigate the effect of operating hours, electricity tariff, ORC unit cost and ORC unit type on the feasibility of the system. According to the results, the electricity tariff and operating hours are the most important parameters because they have a large effect and Play important role on the economic feasibility of the system.     

A feasibility study of solar heating in Iran

A single-glass, flat-plate solar collector for air heating is analyzed for an optimum tilt angle of 45° for Shiraz (29° 36′ N latitude, 52° 32′ E longitude, and elevation of 4500 ft). The absorbed and utilized solar energy, as well as the collector outlet air temperature, the glazing, and the blackened plate temperatures, are determined with respect to the incident solar energy, parametric with collector inlet air temperatures and flow rates and outside air temperature.A 10 ft² collector and an 8 ft³ rock storage are built to experimentally verify the analysis and obtain cost estimates. A 5000 ft² single-story building is considered for solar heating and economic evaluations. Based on an annual interest rate of 8 per cent amortization of the solar heating equipment over 15 yr, electrical energy costs of 3c/kWh, and fuel costs of $1·10 per 10⁶ B.t.u., the optimum collector area which results in minimum annual operating costs (of the solar heating system and the auxiliary heating unit) is determined. A net saving results because solar heating is employed. The feasibility study is extended to eleven other Iranian cities. It is found profitable to employ solar heating in cities with low annual rainfall and relatively cold winters. An effective evaporative cooling is obtained by spraying water over the rock storage during the summer.     

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector (η vs. ΔT_f(0)/I_coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.     

真空管内吸热体接收太阳辐照量的研究

该文对真空管内吸热体分别为平板，圆柱体南向斜置，圆柱体东西横置3种形式在晴天时的日接受太阳辐照量进行了分析研究，推导出了它们与纬度、放置倾角和时间的函数关系，并对北京地区采用的3种安装角度对上述3种形式的集热管的日接收太阳辐照量进行了计算和比较。结果表明，不同纬度地区东西横置圆柱吸热体的日接收太阳辐照量是相同的，有一定倾角的朝南放置圆柱吸热体年平均日接收太阳辐照量高于其东西横置，并且还高于同一倾角的平板吸热体。     

A theoretical study on area compensation for non-directly-south-facing solar collectors

Solar energy integrated with the building is an important approach for the synchronous development of solar energy and architecture. The energy gain of the solar collector integrated with the pitched roof has been greatly influenced by the roof azimuth and tilted angle. Investment cost of the collectors is mainly decided by the size of the collector area. Accordingly, it is significant for solar building design to economically determinate the area compensation of the solar collector at different azimuth and tilted angles. Take Kunming and Beijing as examples, area compensation for the flat-plate tube-fin solar collector used in southern regions and the evacuated tube collector with cylindrical absorbers used in northern regions in China have been theoretically calculated. The results to some extent show that the daily horizontal solar radiation, ambient temperature, the azimuth and tilted angle of the collector integrated into the roof have an influence on the area compensation. The azimuth angle and tilted angle of the roof are the main factors that influence the A/A₀, which is defined as the collector area ratio of the non-south-facing collectors to the south-facing ones with the optimal tilted angle. Comparative studies found that the range of A/A₀ for the evacuated tube collector used in the northern regions is close to that for the flat-plate tube-fin solar collector used in the southern regions. When the pitched roof tilted angle β ∈ [25°, 45°] and the azimuth angle ∣γ∣ ⩽ 30°, the collectors can intercept a lot of solar radiant-energy. Considering the economic situations of the ordinary consumers in China, the optimal area compensation A/A₀ ⩽ 1.30 is recommended in this paper.     

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.     

Payback of solar systems

A variety of solar conversion systems is studied in a dynamic economical model in which the real cost of energy inflates. Payback times and dates of probable market entries are estimated. A distributed system to convert solar energy into heat and electricity in direct proximity to the consumer (Solar One system) is economically attractive even for solar cells with well below 10 per cent conversion efficiency when these can be installed in flat plate collectors for less than $30/m², in addition to the collector cost.     

Experimental study of thermal performance improvement of a solar air flat plate collector through the use of obstacles: application for the drying of ‘yellow onion’

In the solar air flat plate collector, the insufficiency of the thermal exchange between the fluid and the absorber obliges the user to enhance their optimization. This low thermal exchange does not allow these systems to obtain their best performance or the best thermal efficiency. In our experimental study, which consists of a solar energy simulation, we have sought to improve the efficiency–temperature rise couple of the flat plate solar collector by considering several types of obstacles disposed in rows in the dynamic air vein of the collector. Thus, we have proceeded to the application of the best two systems (WDL1 and TL) for drying an agricultural product (Yellow Onion). By comparing with the collector without obstacles (WO), the thermal transfers and, consequently, the output temperature (T_OC) and the collector efficiency (η) are clearly improved. The drying times obtained with the proposed systems are very interesting. The heat quantities obtained in the case of WDL1 are very important compared with the collector WO. However, the entry to the drying cupboard of this high temperature (T_OC) in the vicinity of the solar midday must be limited to the maximal value demanded by the considered product. Copyright © 1999 John Wiley & Sons, Ltd.     

Performance study of a novel solar air collector

A novel solar air collector of pin-fin integrated absorber was designed to increase the thermal efficiency. According to experimental results, the average thermal efficiency of twenty-five kinds of pin-fin arrays collector reach 0.5–0.74 compared to the solar transmittance of 0.83 for the glazing. A correlation equation can be put forward to reflect the maximum thermal efficiency (η_max) of twenty-five kinds of pin-fin arrays collectors as function of dimensionless pin-fin span (s/d) and dimensionless pin-fin height (h/d). By theoretical calculation, the mathematic models of thermal efficiency of twenty-six collectors including flat-plate collector are obtained representing the influences of solar irradiation and inlet conditions of air stream on thermal efficiency. In the performance analysis of varying flow rate on PZ7-11.25 pin-fin arrays collector, the correlation equation on heat transfer coefficient is obtained and the efficiency variation vs. air flow rate is determined in this work.