Collector Loss Coefficient Research Articles

Thermosyphon heat-pipe evacuated tube solar water heaters for northern maritime climates

► This work demonstrated that such systems are technologically feasible for this climate. ► A simplifying assumption gave a statistically significant relationship. ► Assumption applied to a different system and relationship still statistically significant. Under transient climatic conditions previous research has reported that evacuated tube solar water heaters (ETSWHs) with heat-pipe absorbers are the most effective solution for collection of solar energy. The cost of such systems is greater than the mass produced “water in glass” evacuated tube solar water heater mainly manufactured in China. Previous studies have reported that the costs of solar water heating can be reduced through the adoption of thermosyphon fluid circulation. Well designed thermosyphon systems are as effective as pumped systems but with lower capital and running costs. To investigate if costs could be reduced and performance levels maintained, outdoor testing of three thermosyphon heat-pipe ETSWHs primarily designed for pumped fluid circulation was carried out under a northern maritime climate. Experimental data from a year’s side by side monitoring of two thermosyphon ETSWHs (both with the same area of 2 m 2 ) was collected and used to validate a correlation based on a modified version of the f -chart design tool between the observed and expected performance for both systems. The R 2 value between measured and predicted monthly solar fractions was greater than 0.99 for both systems. The R 2 value between measured and predicted diurnal solar fractions was calculated as greater than 0.95 for both systems. The only difference between the two was that one utilised internal heat-pipe condensers whilst the other used external ones. The system with internal condensers was found to be 17% more efficient. A simplifying assumption of a constant temperature rise across the collectors reduced the calculations required to predict the performance of thermosyphon heat-pipe ETSWHs and was also statistically significant. To determine if the assumption was valid for other thermosyphon heat-pipe ETSWHs with different collector parameters a third system with internal condensers an area of 3 m 2 , a heat removal factor ( F R ) of 0.816 based on the absorber area and a collector loss coefficient ( F R U L ) of 2.25 W m −2 K −1 was assembled and its performance monitored, when the same assumption was applied the R 2 value between the measured and predicted daily solar fractions was calculated as 0.96 experimentally demonstrating that this relationship was still statistically significant for another heat-pipe thermosyphon ETSWH with different collector parameters.

An analytical study on the thermal characteristics of flat-plate and evacuated solar collectors

In this study, we highlighted differences in the standards used in performance tests of solar collectors. We analyzed testing results for different types of solar collectors to determine the effects of the collector area and mass flow rate, which were not necessarily consistent across all tests. Our analysis showed that the factor,F′ (τα), including collector efficiency factor (F′), could be correlated with the flow rate or area regardless of the collector type. Moreover, the collector loss coefficient (F′U L ) per flow rate or area for an evacuated collector was less that of a flat-plate collector; this was also correlated with the flow rate or area, regardless of the type of evacuated collector. As a result of this analysis, we propose a modified heat loss coefficient that includes the effects of all parameters that can be considered in a performance test and show that this coefficient could better describe the thermal characteristics of various types of solar collectors.

Back absorbing parallel plate polymer absorbers in solar collector design

Serious efforts are currently devoted towards making solar water heating technology economically competitive. Among them, the development of large, all plastic, innovative design, low cost collector modules suitable for mass production is one of the most promising, which will eventually lead to a drastic reduction of the solar collector subsystem cost. Although appreciable development work was earlier devoted towards production of all plastic solar collectors, the polymer absorber still remains the most crucial part of the polymer collector design, since severe limitations are imposed by the low thermal conductivity of polymers. To overcome these restrictions, a suitable design should be implemented, allowing extended wetted surfaces of the absorbers, something which leads to the familiar extruded parallel polymer plate absorber design. Recent investigations have been directed toward definition of the limitations and development of design criteria for the top absorbing parallel polymer plate absorber. In the present work, a subsequent analysis is developed for evaluation of the back absorbing polymer plate design, which allows the absorption of radiation at the water stream and the back plate. The results of the current analysis, which are presented comparatively to those obtained for the top absorbing plate design, show a remarkable improvement of the collector efficiency factor and a corresponding reduction of the collector loss coefficient, something which leads to a significant improvement of the instantaneous heat collection efficiency of collectors using a back absorbing plate absorber design.

A generalised steady state collector model including pipe losses, heat exchangers, and pump powers

Abstract A generalised quasi-steady-state collector model is derived which includes the effects of pipe losses, one or more circulation pumps, and either an internal or an external collector-store heat exchanger. This leads to expressions for the increased effective collector loss coefficient, and the increased de Winter heat exchanger penalty factor. These increased loss factors are shown to be coupled, strongly, to the quality of the pipe insulation. This coupling is not indicated in the standard textbook descriptions. Proposed European solar water heating test standards make use of the international standard ISO 9459-5 for complete system testing using a dynamic system test. The links are derived herein which show the relationships between the physical system parameters and the ‘non-physical’ model parameters obtained by the ISO 9459-5 system tests. It is seen from the theory that separating the pipe losses from the collector losses is not possible without substantial additional control of the test conditions, namely the ambient temperature surrounding the pipes.

Thermal and optical consequences of the introduction of baffles into compound parabolic concentrating solar energy collector cavities

A theoretical and experimental investigation into the modifications in optical and thermal performance resulting from the introduction of a baffle into the cavity of a compound parabolic concentrating solar energy collector has been performed. Results are presented in the form of velocity vector diagrams and isothermal plots. A comparison is made of the collector loss coefficient and Hottel-Whiller-Bliss graphs are produced for cavities with and without a baffle present. The introduction of a baffle reduces internal convection thereby reducing heat losses. The associated reduction in optical efficiency is small.

Thermal and optical consequences of the introduction of baffles into compound parabolic concentrating solar energy collector cavities

A theoretical and experimental investigation into the modifications in optical and thermal performance resulting from the introduction of a baffle into the cavity of a compound parabolic concentrating solar energy collector has been performed. Results are presented in the form of velocity vector diagrams and isothermal plots. A comparison is made of the collector loss coefficient and Hottel-Whiller-Bliss graphs are produced for cavities with and without a baffle present. The introduction of a baffle reduces internal convection thereby reducing heat losses. The associated reduction in optical efficiency is small.

A Design Method for Thermosyphon Solar Domestic Hot Water Systems

A modification to the f-Chart method has been developed to predict monthly and annual performance of thermosyphon solar domestic hot water systems. Stratification in the storage tank is accounted for through use of a modified collector loss coefficient. The varying flow rate throughout the day and year in a thermosyphon system is accounted for through use of a fixed monthly “equivalent average” flow rate. The “equivalent average” flow rate is that which balances the thermosyphon buoyancy driving force with the frictional losses in the flow circuit on a monthly average basis. Comparison between the annual solar fraction predited by the modified design method and TRNSYS simulations for a wide range of thermosyphon systems shows an RMS error of 2.6 percent.

Prediction of Solar Air Heater Thermal Performance

ABSTRACT THE prediction of steady state thermal performance of solar air heaters requires prediction of three parameters: the effective transmittance-absorptance product, the collector heat removal factor and the collector loss coefficient. Agreement between computed values and measured values of these parameters indicated that the first two parameters could be adequately predicted for use in collector design, but the collector loss coefficient predictions did not agree with measured values. The measured values consisted of a steady-state thermal performance test using the ASHRAE Standard 93-77 as a guide, this same steady-state test but additionally using measured average plate temperature for computing the loss coefficient, a cloudless sky night test using heated air input to achieve approximately the same plate temperature and a computed value of the loss coefficient using a measured top loss coefficient from a similar absorber plate which was electically heated.

Sensitivity of Air-Type Solar Collector Efficiency to Design Changes

ABSTRACT THE sensitivity of the thermal efficiency, the collector efficiency factor and the collector loss coefficient, to design parameter (top loss, bottom loss, convective and radiative heat transfer in the collector) changes is presented graphically for three common air-type solar collectors. Interactions are shown to exist as design parameters change. Design parameter changes have different effects depending on location of the flow channel. The most critical component of air-type solar collector design is the convective heat transfer coefficient in the flow channel since its effect on efficiency may be positive or negative depending on collector type and operating conditions.

The effect of inclination on the heat loss from flat-plate solar collectors

One of the many design variables that affects the heat losses from flat-plate solar collectors is the angle of inclination of the collectors to the horizontal. This is due to the variation in natural convection conductances in spaces between flat plates, with their angle to the horizontal. The top loss heat transfer coefficient is calculated for a series of plate temperatures, ambient temperatures, external convective heat transfer coefficients and plate emittances for angles of inclination from 0 to 90° using the natural convection correlation developed by Hollands et al.[4]. A sky temperature 12°C below ambient temperature is used as the radiant sink temperature and an effective sink temperature for the top losses is defined. Curves are presented showing the variation of the top loss coefficient with temperature and wind speed for two plate emittances at an angle of inclination of 45°. It is shown that the value of the top loss coefficient is insensitive to the effective sink temperature (as found by Duffie and Beckman [5]) and that the effective temperature is determined solely by the wind speed, for a given collector inclination. The top loss coefficient at any angle of inclination is expressed as a ratio of the top loss coefficient at 45°. The results indicate that there is a continual reduction in the top loss coefficient up to an inclination of 90°. The effect this has on the overall collector loss coefficient is illustrated and the change in collector instantaneous efficiency is estimated.

Design Equations for Solar Air Heaters

ABSTRACT THE Hottel-Whillier-Bliss generalized performance equation is probably the best available mathematical description for the performance of a flat plate collector. The most useful form of this equation utilizes the collec-tor heat removal factor and the collector loss coefficient. The collector heat removal factor can be computed from the collector efficiency factor, collector loss coefficient and the flow rate. The purpose of this paper is to present the mathematical formulation for computing collector efficiency and output temperature using derived equa-tions for the collector loss coefficient and the collector ef-ficiency factor. The equations for collector efficiency fac-tor and loss coefficient in this paper provide the basis for analytical prediction of collector performance when used in the Hottel-Whillier-Bliss Equation.

Simulation study of several solar heating systems with offpeak auxiliary

In essentially all climates, solar heating systems for buildings must incorporate back-up conventional (auxiliary) heating equipment sized to meet the maximum load of the building. In areas with high densities of solar heated buildings using auxiliary energy supplied by utilities, unacceptable peaks in auxiliary energy demand (during cloudy weather) can be prevented via limited on-site storage of the utility supplied energy. In this paper several ''offpeak auxiliary'' solar air heating systems are compared using simulation methods. Two basic types of offpeak systems have been studied; Systems I and II, which separate the storage of auxiliary and solar energy, and Systems III and IV, which store all energy in the same pebble bed. Simulation methods are used to determine the effect of system configuration, collector loss coefficient, controls, storage size, and collector flow rate on system performance.