Optimum outlet temperature of solar collector for maximum work output for an Otto air-standard cycle with ideal regeneration

The optimum solar collector outlet temperature for maximizing the work output for an Otto air-standard cycle with ideal regeneration is investigated. A mathematical model for the energy balance on the solar collector along with the useful work output and the thermal efficiency of the Otto air-standard cycle with ideal regeneration is developed. The optimum solar collector outlet temperature for maximum work output is determined. The effect of radiative and convective heat losses from the solar collector, on the optimum outlet temperature is presented. The results reveal that the highest solar collector outlet temperature and, therefore, greatest Otto cycle efficiency and work output can be attained with the lowest values of radiative and convective heat losses. Moreover, high cycle work output (as a fraction of absorbed solar energy) and high efficiency of an Otto heat engine with ideal regeneration, driven by a solar collector system, can be attained with low compression ratio.     

Operation and evaluation of the Willard solar thermal power irrigation system

The operation of the Willard solar thermal power system is analyzed and evaluated. The 19 kW (25 hp) power system was coupled to a shallow well and sprinkler system near Willard, New Mexico irrigating approximately, 49 hectares. The specific performance of the major subsystems—collector array, thermal storage, and the organic working fluid Rankine cycle heat engine—were determined. Over the summer months, the daily collector array efficiency (based on direct solar radiation normalized in the plane of collector aperature) was nominally 25 per cent and heat engine rankine cycle efficiency 15 per cent. These conversion efficiencies coupled with the numerous system losses resulted in an overall efficiency of nearly 3 per cent on clear summer days. Electrical parasitic losses reduced the system's net power output by about 20 per cent on clear days and greater amounts on other days. The maintenance and repair effort was distributed evenly among the collector array and the heat engine.     

Maximum power-conversion efficiency for the utilization of solar energy

The overall efficiency of solar thermal power plants is investigated for estimating the upper limit of their practical performances. This study consists of the theoretical optimization of the heat engine and the optimization of the overall system efficiency, which is the product of the efficiency of the solar collector and the efficiency of the heat engine. In order to obtain a more realistic performance of the solar thermal power plant, the solar collector concentration ratio, the diffused solar radiation and the convective and radiative heat losses of the solar collector are taken into account. Instead of the classical Carnot efficiency, the efficiency at maximum power is used as the optimal conversion efficiency of a heat engine. By means of simple calculations, the optimal overall system efficiency and the corresponding operating conditions of the solar collector are obtained. The results of the present work provide an accurate guide to the performance estimation and the design of solar thermal power plants.     

Exergetic analysis of a solar thermal power system

This communication presents a second law analysis based on an exergy concept for a solar thermal power system. Basic energy and exergy analysis for the system components (viz. parabolic trough collector/receiver and Rankine heat engine, etc.) are carried out for evaluating the respective losses as well as exergetic efficiency for typical solar thermal power systems under given operating conditions. It is found that the main energy loss takes place at the condenser of the heat engine part, whereas the exergy analysis shows that the collector–receiver assembly is the part where the losses are maximum. The analysis and results can be used for evaluating the component irreversibilities which can also explain the deviation between the actual efficiency and ideal efficiency of a solar thermal power system.     

Model of optimized solar heat engine operating on Mars

An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performance. The output power is maximized. Meteorological and actinometric data provided by the Viking Lander 1 are used as inputs. Four strategies of collecting solar energy are considered. Results concerning the following three parameters are briefly reported: (1) optimum solar collector surface area, (2) optimum solar collector temperature and (3) maximum output power.     

Efficiency bound of a solar-driven stirling heat engine system

A solar-driven Stirling engine is modelled as a combined system which consists of a solar collector and a Stirling engine. The performance of the system is investigated, based on the linearized heat loss model of the solar collector and the irreverisible cycle model of the Stirling engine affected by finite-rate heat transfer and regenerative losses. The maximum efficiency of the system and the optimal operating temperature of the solar collector are determined. Moreover, it is pointed out that the investigation method in the present paper is valid for other heat loss models of the solar collector as well, and the results obtained are also valid for a solar-driven Ericsson engine system using an ideal gas as its engine work substance. © 1998 John Wiley & Sons, Ltd.     

SECOND LAW ANALYSIS OF A SOLAR THERMAL POWER SYSTEM

This communication presents second law analysis based on exergy concept for a solar thermal power system. Basic energy and exergy analysis for the system components (viz. parabolic trough collector/receiver and Rankine heat engine etc.) are carried out for evaluating the energy and exergy losses as well as exergetic efficiency for typical solar thermal power system under given operating conditions. Relevant energy flow and exergy flow diagrams are drawn to show the various thermodynamic and thermal losses. It is found that the main energy loss takes place at the condenser of the heat engine part whereas the exergy analysis shows that the collector-receiver assembly is the part where the losses are maximum. The analysis and results can be used for evaluating the component irreversibilities which can also explain the deviation between the actual efficiency and ideal efficiency of solar thermal power system.     

Parametric optimization of a solar-driven Braysson heat engine with variable heat capacity of the working fluid and radiation–convection heat losses

An irreversible solar-driven Braysson heat engine system is presented, in which the temperature-dependent heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the irreversibilities resulting from heat transfer and non-isentropic compression and expansion processes are taken into account. Based on the thermodynamic analysis method and the optimal control theory, the mathematical expression of the overall efficiency of the system is derived and the maximum overall efficiency is calculated, and the operating temperatures of the solar collector and the cyclic working fluid and the ratio of heat-transfer areas of the heat engine are optimized. By using numerical optimization technology, the influences of the variable heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the multi-irreversibilities on the performance characteristics of the solar-driven heat engine system are investigated and evaluated in detail. Moreover, it is expounded that the optimal performance and important parametric bounds of the irreversible solar-driven Braysson heat engine with the constant heat capacity of the working fluid and the irreversible solar-driven Carnot heat engine can be deduced from the conclusions in the present paper.     

On optimized solar-driven heat engines

Heat engines will usually be designed somewhere between the two limits of (1) maximum efficiency, which corresponds to “Carnot” or reversible operation, albeit at zero power, and (2) maximum power point. Each of these limits implies a specific dependence of heat engine efficiency on the temperatures of the hot and cold reservoirs between which the heat engine operates. We illustrate that the energetically optimal operating temperature for solar-driven heat engines is relatively insensitive to the engine design point. This also pertains to solar collectors whose heat loss can range from predominantly linear (conductive/convective) to primarily radiative. Potential misconceptions are also discussed regarding the maximum power point and the Curzon-Ahlborn efficiency of “finite-time thermodynamics.”     

Optimum performance characteristics of an irreversible solar-driven Brayton heat engine at the maximum overall efficiency

An irreversible cycle model of a solar-driven Brayton heat engine is established, in which the heat losses of the solar collector and the external and internal irreversibilities of the heat engine are taken into account, and used to investigate the optimal performance of the cycle system. The maximum overall efficiency of the system is determined. The operating temperature of the solar collector and the temperature ratio in the isobaric process are optimized. The influence of the heat losses of the solar collector and the external and internal irreversibilities of the heat engine on the cyclic performance is discussed in detail. Some important curves which can reveal the optimum performance characteristics of the system are given. The results obtained here are general, and consequently, may be directly used to discuss the optimal performance of other solar-driven heat engines.     

Optimum solar collector operation for maximizing cycle work output

Analysis is presented to determine the optimum outlet temperature for a given solar collector that will maximize the work output for various idealized heat engine cycles. The effect of radiative and convective heat losses from the collector is demonstrated, and the relative importance of each in different operating ranges is shown.     

On the climatic optimization of the tilt and azimuth of flat-plate solar collectors

A numerical climatic model for computing total solar irradiance on the surface of a flat-plate collector, positioned at any tilt and azimuth, is described. Owing to a small time-step (one hour), and a quasi-realistic characterization of a collector's environment, the algorithm is able to produce credible estimates of both the climatically “optimal” position and the amount of energy lost to a collector when it is non-optimally positioned. Exemplary computations for Sterling, Virginia and Sunnyvale, California are presented and they suggest that the non-optimal positioning of a collector, e.g. as a simple function of latitude and a few highly summarized climatic-environmental variables, will not, in many cases, produce significant losses of available solar irradiance. In other situations, however, where a collector's horizon is significantly obstructed and/or the climatic environment of the area creates large diurnal or seasonal asymmetries in available irradiance, non-optimal positioning may cause sizeable energy losses. It is also apparent that even moderately sized horizonal obstructions, which are “seen” by a collector, can substantially reduce the amount of available irradiance, relative to an unobstructed horizon.     

Optimization of Output Power and Thermal Efficiency of Solar‐Dish Stirling Engine Using Finite Time Thermodynamic Analysis

This paper presents an investigation on finite time thermodynamic (FTT) evaluation of a solar‐dish Stirling heat engine. FTTs has been applied to determine the output power and the corresponding thermal efficiency, exergetic efficiency, and the rate of entropy generation of a solar Stirling system with a finite rate of heat transfer, regenerative heat loss, conductive thermal bridging loss, and finite regeneration process time. Further imperfect performance of the dish collector and convective/radiative heat transfer mechanisms in the hot end as well as the convective heat transfer in the heat sink of the engine are considered in the developed model. The output power of the engine is maximized while the highest temperature of the engine is considered as a design parameter. In addition, thermal efficiency, exergetic efficiency, and the rate of entropy generation corresponding to the optimum value of the output power is evaluated. Results imply that the optimized absorber temperature is some where between 850 K and 1000 K. Sensitivity of results against variations of the system parameters are studied in detail. The present analysis provides a good theoretical guidance for the designing of dish collectors and operating the Stirling heat engine system.     

Performance of a “black” liquid flat-plate solar collector

A cost-effect, “black” liquid, flat-plate solar collector has been designed, and prototypes have been built and tested. In these collectors a highly absorbent “black” liquid flows in transparent channels and directly absorbs solar energy. The liquid is the hottest substance in the collector, and no metals are required anywhere in the design. The collector differs in the following ways from conventional flat-plate collectors:

1. 1. Solar radiation is absorbed directly by the black liquid without the need to heat any other structures within the collector.

2. 2. Lower heat losses are possible since energy is absorbed directly by the working fluid, and the flow pattern can be arranged so that the hottest spot is in the center of the collector away from all edges. As the fluid moves progressively inward toward the exit, which is located at the center of the collector, it will pick up some of the heat loss along the radial direction.

3. 3. Lower cost may be possible since no metal is required in construction and only glass and/or plastic need be used in addition to the insulation and frame. The absence of metal should eliminate all corrosion problems.

4. 4. New avenues of research are opened up by the use of black liquids: an entirely new class of materials are available which may aid in finding inexpensive, durable absorbers.

5. 5. New configurational arrangements are possible with the absence of metal absorbers.

Experimental performance data for the black liquid collector is presented which compares favorably with other conventional flat-plate collectors.     

Flat-plate solar-collector performance evaluation with a solar simulator as a basis for collector selection and performance prediction

The use of a solar simulator for performance determination permits collector testing under standard conditions of wind, ambient temperature, flow rate and “Sun”. The performance results determined with the simulator have been found to be in good agreement with outdoor performance results.This paper reports the measured thermal efficiency and evaluation of 23 collectors which differ according to absorber material (copper, aluminum, steel), absorber coating (nonselective black paint, selective copper oxide, selective black nickel, selective black chrome), type of glazing material (glass, Tedlar, Lexan, anti-reflection glass), the use of honeycomb material and the use of vacuum to reduce thermal convection losses. The collectors are given performance rankings based on noon-hour solar conditions and all-day solar conditions. The determination with the simulator of an all-day collector performance is made possible by tests at different incident angles. The solar performance rankings are made based on whether the collector is to be used for pool heating, hot water, absorption air conditioning, heating, or for a solar Rankine machine.Another test which aids in selecting collectors is a collector heat capacity test. This test permits a ranking of collectors according to their heat capacity (and time constant), which is a measure of the rapidity of a collector's response to transient solar conditions. Results are presented for such tests.Final considerations for collector selection would of course be made on the basis of cost and the reliability of performance over the required life of a collector. Results of a cost-effectiveness study is given for conditions corresponding to those required for absorption or heating. These results indicate that the additional cost involved in the upgrading of collector performance (selective surfaces, anti-reflection glass, etc.) appears to be cost effective and therefore justified. Some data are also presented to illustrate a method for the determination of outdoor performance degradation by use of simulator tests carried out before and after a period of outdoor operation.     

Energy losses through entrance condensation in small vapour engines

The effects of entrance condensation were studied in a small piston type vapour engine as could be used for low power thermodynamic solar waterpumping (50–1000 W output). Indicative relations have been established between the magnitude of energy losses caused by this phenomenon and engine design features.     

Solar houses/heating and cooling progress report

Performance data on seven solar homes are given. Solar Homes No. 1, 2, 3, and 4 are near Washington, D.C., 39° north latitude, where about half of the winter days are cloudy and temperatures drop far below freezing, sometimes to 0°F. These houses are described in the book Solar Houses and Solar House Models by Harry E. Thomason, published by Edmund Scientific Company, Barrington, New Jersey, 08007. Edmund Scientific Co. also publishes Solar House Plans, for building a house similar to Solar House No. 1, with improvements.

1. Solar House No. 1Solar House No. 1 has been in continuous operation for thirteen years. In its first year, solar heat supplied about 95 per cent of the heat requirements for home temperatures at 70°F, plus or minus 2°F. After 5 yr of operation, the heat collector was rebuilt. Longer-lived materials were used although efficiency was lowered somewhat. Also changes were made in the air conditioning system.

2. Solar House No. 2A number of changes were incorporated in House No. 2, built in 1960 and 1961. Cost for the original system was lowered, but the auxiliary heat cost ran slightly higher. An aluminum reflector was installed at the bottom of the solar heat collector to reflect additional sunlight onto the collector. The air conditioning system in House No. 2 is rather satisfactory, and that type of system is now in House No. 1 as well as in House No. 2.Summertime heat leakage from the solar heat collector into the closet space behind the collector was solved in House No. 2. The closet remains cool. However, at times the temperature in the closet drops too low and a new problem has to be solved by re-introducing heat to the closet.

3. Solar House No. 3The architectural appearance of House No. 3 was improved. Low-cost glazing with a minimum of glass breakage was achieved. The heat collector was moved entirely up to the roof so that winter sunshine enters the living room and built-in swimming pool on the south side. Improved air conditioning was installed.

4. Solar House No. 4A new type of solar heat collector (with asphalt shingles) and a new type of low-cost “Pancake” heat storage were incorporated into this A-frame house.

5. Solar House No. 5House No. 5, planned for a South Carolina firm, was never built due to insufficient funds.

6. Solar House No. 6House No. 6 has been completed in Mexico City, Mexico. The house and system were not constructed as the authors recommended so the solar heating system does not provide the major part of the heat load. Although Mexico City is quite far south (19° north latitude), the temperature drops below the freezing mark at times. (On December 8, 1970, the temperature dropped to 24°F.)

7. Solar House No. 7The authors have designed and engineered a new system, their Solaris “Sunny South Model,” with three principle innovations. (1) The system uses “Pancake” under-the-floor heat storage. (2) The system utilizes a shallow roof-pond solar heat collector with a reflector to intensify solar input. (3) The system allows the warm water from the roof-pond to drain each night to the under-floor “Pancake” heat storage area where it warms the floor and living space. During the summer the roof-pond helps minimize day-night temperature extremes by absorbing excess heat during the day and liberating it at night.

     

Field performance and operation of a flat-glass solar heat collector

A “Base-Line,” flat-glass solar heat collector has been designed and constructed that can be manufactured economically for commercial use. Four of the collectors, 34 by 76 in. (approximately 18 ft²), were installed to provide hot water to a private home in Melbourne, Florida.The details of the collector are described, including coverplates, solar absorber, absorber coating, spacers, seals and glazing.A simple relationship has been established between the collector efficiency, the collector temperature and the rate of insolation for constant rates of flow of circulating fluids.The theoretical and field performance curves have been correlated for collector efficiency, collector temperatures, incident solar radiation and ambient air temperatures. The effect of fluid flow on collector temperatures for various collector parameters has also been presented.     

Experimental study on the performance of solar Rankine system using supercritical CO2

An experimental study is carried out to investigate the performance of a solar Rankine system using supercritical CO₂ as a working fluid. The testing machine of the solar Rankine system consists of an evacuated solar collector, a pressure relief valve, heat exchangers and CO₂ feed pump, etc. The solar energy powered system can provide electricity output as well as heat supply/refrigeration, etc. The system performance is evaluated based on daily, monthly and yearly experiment data. The results obtained show that heat collection efficiency for the CO₂-based solar collector is measured at 65.0–70.0%. The power generation efficiency is found at 8.78–9.45%, which is higher than the value 8.20% of a solar cell. The result presents a potential future for the solar powered CO₂ Rankine system to be used as distributed energy supply system for buildings or others.