Simon Furbo

Thermal advantages for solar heating systems with a glass cover with antireflection surfaces

Abstract Investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5–9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4–6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces.

Theoretical Comparison of Solar Water/Space-Heating Combi Systems and Stratification Design Options

A theoretical analysis of differently designed solar combi systems is performed with weather data from the Danish Design Reference Year (55 deg N). Three solar combi system designs found on the market are investigated. The investigation focuses on the influence of stratification on the thermal performance under different operation conditions with different domestic hot water and space heating demands. The solar combi systems are initially equipped with heat exchanger spirals and direct inlets to the tank. A step-by-step investigation is performed demonstrating the influence on the thermal performance of using inlet stratification pipes at the different inlets. Also, how the design of the space heating system, the control system of the solar collectors, and the system size influence the thermal performance of solar combi systems are investigated. The work is carried out within the Solar Heating and Cooling Programme of the International Energy Agency (IEA SHC), Task 32.

Correlation of experimental and theoretical heat transfer in mantle tanks used in low flow SDHW systems

Experimental and theoretical investigations of vertical mantle tanks for solar domestic hot water systems have been carried out. Differently designed mantle tanks have been evaluated in a laboratory test facility and a transient three-dimensional CFD-model of one of the mantle tanks is developed in the CFD-program CFX 4.1. The model is validated against the experimental tests and good agreement between measured and calculated results are achieved. The results from the CFD-calculations are used to illustrate the thermal behaviour and the fluid dynamics in the mantle and in the inner tank. The CFD-calculations are used to carry out a detailed analysis of the heat transfer from the solar collector fluid to the wall of the inner tank. The analysis has resulted in a local Nusselt-Rayleigh correlation for the heat transfer between the solar collector fluid and the wall of the inner tank.

IS LOW FLOW OPERATION AN ADVANTAGE FOR SOLAR HEATING SYSTEMS

ABSTRACT The thermal performance of three different small solar heating systems for domestic hot water supply has been measured under the same conditions. A reference system with a normal volume flow rate, a pumped system using a low volume flow rate, and a thermosyphon system have been tested. The pumped system using low flow performs about 20% better than the reference system and 10% better than the thermosyphon system.

Buoyancy Effects on Thermal Behavior of a Flat-Plate Solar Collector

Theoretical and experimental investigations of the flow and temperature distribution in a 12.53 m 2 solar collector panel with an absorber consisting of two vertical manifolds interconnected by 16 parallel horizontal fins have been carried out. The investigations are focused on overheating and boiling problems in the collector panel. Single-phase liquid flow and heat transfer in the collector panel are studied by means of computational fluid dynamics (CFD) calculations. Differently designed collectors are investigated with different collector fluid volume flow rates. The effect of friction and the influence of the buoyancy effects are considered in the investigations. Further, experimental investigations of the solar collector panel are carried out. The flow distribution through the absorber is evaluated by means of temperature measurements on the back of the absorber tubes. The measured temperatures are compared to the temperatures determined by the CFD model and there is a good agreement between the measured and calculated temperatures. Calculations with the CFD model elucidate the flow and temperature distribution in the collector. The influences ofcollector fluid flow rate and inlet temperature on the flow and temperature distribution are shown. The flow distribution through the absorber tubes is uniform if a high flow rate of 10.0 1/min is used. By decreased collector fluid flow rate and by increased collector fluid inlet temperature, the flow distribution gets less uniform due to the influence of buoyancy force. If the collector fluid flow rate is small and the collector fluid inlet temperature is high enough, severe nonuniform flow distribution may happen with a small flow rate or even zero or reverse flow in the upper horizontal strips, resulting in overheating or boiling problems in the strips. The CFD calculations elucidate the flow and temperature distribution in the collector panels of different designs. Based on the investigations, recommendations are given in order to avoid overheating or boiling problems in the solar collector panel.

Thermal performance of a large low flow solar heating system with a highly thermally stratified tank

In year 2000 a 336 m 2 solar domestic hot water system was built in Sundparken, Elsinore, Denmark. The solar heating system is a low flow system with a 10000 1 hot-water tank. Due to the orientation of the buildings half of the solar collectors are facing east. half of the solar collectors are facing west. The collector tilt is 15° from horizontal for all collectors. Both the east-facing and the west-facing collectors have their own solar collector loop, circulation pump, external heat exchanger and control system. The external heat exchangers are used to transfer the heat from the solar collector fluid to the domestic water The domestic water is pumped from the bottom of the hot-water tank to the heat exchanger and back to the hot-water tank through stratification inlet pipes. The return flow from the DHW circulation pipe also enters the tank through stratification inlet pipes. The tank design ensures an excellent thermal stratification in the tank. The solar heating system was installed in May 2000. The thermal performance of the solar heating system has been measured in the first two years of operation. Compared to other large Danish solar domestic hot water systems the system is performing well in spite of the fact that the solar collectors are far from being orientated optimally. The utilization of the solar radiation on the collectors is higher, 46% in the second year of operation, than for any other system earlier investigated in Denmark, 16%-34%. The reason for the good thermal performance and for the excellent utilization of the solar radiation is the high hot-water consumption and the good system design making use of external heat exchangers and stratification inlet pipes.

Thermal performance of Danish solar combi systems in practice and in theory

An overview of measured thermal performances of Danish solar combi systems in practice is given. The thermal performance varies greatly from system to system. Measured and calculated thermal performances of different solar combi systems are compared and the main reasons for the different thermal performances are given. Further, a parametric study on two solar combi system types is performed. Based on the investigation it can be concluded that the thermal performance first of all is influenced by the space heating consumption during the summer period and that the systems in practice perform as theoretically expected.

CALCULATION OF THE THERMAL PERFORMANCE OF SMALL HOT WATER SOLAR HEATING SYSTEMS USING LOW FLOW OPERATION

ABSTRACT Experiments have shown that small solar heating systems using low flow operation and a hot water tank with an enclosing mantle perform extremely well. A detailed mathematical model simulating the thermal behaviour of such solar heating systems has been developed and validated by means of indoor experiments with a mantle heat storage. The yearly thermal performance of different designs of and operation strategies for small low flow solar heating systems have been calculated. By use of the model it is possible to optimize the design of the system, the operation and control strategy of the system. The calculated thermal performance of the low flow system is compared to the calculated thermal performance of a traditional solar heating system using a normal flow rate. The calculated increase of thermal performance of the low flow system is somewhat smaller than the measured increase of the thermal performance. Therefore, work is still necessary in order to validate the developed model by means of experiments with a complete solar heating system.

Solar Heating Systems With Heat of Fusion Storage With 100% Solar Fraction for Solar Low Energy Builidngs

A storage concept based on the phase change material (PCM) sodium acetate trihydrate and active use of supercooling is theoretically investigated by means of TRNSYS simulations. The supercooling makes it possible to obtain a partly heat loss free storage when the melted salt due to heat loss supercools to the surrounding temperature and no further heat loss occur. The heat of fusion energy is preserved and can be released by activation of the solidification on demand. The investigations show that 100% solar fraction can be reached in a low energy house in a Danish climate with a solar collector area of 36 m2 and a PCM storage volume of 6 m3 combined with a 180 litres DHW tank. Experiments have proved that it is possible to melt large volumes of sodium acetate and an automatic controllable activation mechanism has been developed and tested.

Modeling Shadows on Evacuated Tubular Collectors With Cylindrical Absorbers

A new TRNSYS collector model for evacuated tubular collectors with tubular absorbers is developed. Traditional flat plate collector performance equations have been integrated over the whole absorber circumference. On each tube the model determines the size and position of the shadows caused by the neighbor tube. An all glass tubular collector with tubular absorbers with 14 tubes connected in parallel is investigated theoretically with the model and experimentally in an outdoor collector test facility. Performance calculations with the new model are compared with measured results and a good degree of similarity between the measured and calculated results is found. Further, it is illustrated how the model can be used for geometrical parameter studies both for constant collector mean fluid temperatures and for varying temperature conditions in a solar heating plant. These investigations are performed for two climates: Copenhagen (Denmark) and Uummannaq (Greenland).