Pipe internal recirculation in storage connections - Characteristics and influencing parameters

Pipe internal recirculation (PIR) is an undesired phenomenon which may lead to significant additional heat losses. Although different research institutes already measured singular PIR effects, there is still a lack of wide-ranging evaluations and practical solutions. This paper describes a test and evaluation method and reports on results regarding the influence of the pipe's slope, the material (copper, stainless steel) and diameter on the PIR caused heat loss coefficient. For combined pipe path arrangements a new method is presented, which allows to calculate the heat losses from basic investigations on horizontal and vertical pipe orientations. This method has been experimentally validated. Finally, the effectiveness of different measures for the reduction of PIR induced heat losses is presented

Simulation and Evaluation of Solar Thermal Combi Systems with Direct Integration of Solar Heat into the Space Heating Loop

Usually, solar heat in combi systems is used via a buffer storage. In contrast to that, the solar collectors may be connected directly to the space heating circuit in order to store the heat in the building itself. Such a direct solar integration is investigated within system simulations for different layouts and heating elements. The simulations show significant reductions in the final energy demand as well as an increasing solar yield due to less thermal losses of the storage tank compared to the usual solution with one buffer storage. A prototype of one of the investigated heating concepts within a single family house proofs the functionality of the system concept and the high solar yield, particularly at low radiation levels. Since only a few manufacturers provide such system solutions with a direct solar integration, the results may have an important impact on the future development of combi systems

Thermal storage tanks in high efficiency heat pump systems - Optimized installation and operation parameters

Heat pumps have a significant and increasing share in the European heating market. In most applications heat pumps are operated with a storage tank, either for domestic hot water or for the space heating circuit. The design of the storage and its connection to the heat pump has a significant influence on the performance of the heat pump and the whole system. Within a comprehensive investigation, system simulations in TRNSYS are used to identify the optimum design of two typical systems with a heat pump connected to a buffer storage including total storage volume, number and size of heated zones, sensor and in- and outlet positions. As result, a sharp decrease in the energy demand can be reached if the storage is equipped with two heat zones instead of one. This can be explained by the decreased set temperature in the additional heat zone for space heating leading to a higher performance of the heat pump. Other measures with a positive effect on the energy demand are a large space heating volume, the number and place of sensors, low set temperatures and the sensor position in the upper part of the heating zone. The investigation gives important recommendations how to connect a heat pump with a storage tank and may be considered as design rules for all heating systems consisting of heat pump and different types of storage tanks

Performance and reliability of a high efficiency flat plate collector - Final results on prototypes

The paper presents the final results of our prototype investigations regarding the performance and reliability of a high efficiency flat plate collector with a gas-filled low-e double-glazing as transparent cover. The measured efficiency of an optimized prototype results in an increase in efficiency of 70 % at a temperature difference of 60 K (G = 500 W/m2) compared to a standard flat plate collector. The effect of additional antireflecting coatings and different gas fillings of the glazing is analyzed by prototype measurements. As the heat losses of the collector are significantly reduced by the low-e glazing, a stagnation temperature of 265 °C was determined at the absorber plate experimentally. The collector components have to withstand these increased temperature loads. Test and assessment methods for the long term stability of the hermetically sealed double-glazing against high temperatures and the combination of temperature and UV-radiation were developed and performed successfully. Further, the reliability of four prototype collectors was confirmed by a one-year outdoor-exposure test. A simplified economic analysis resulted in an optimum operating range for the collector of 80 °C to 110 °C compared to commercially available flat plate and evacuated tube collectors. As the main result of the research project, a new flat plate collector with a significantly increased performance and an experimentally proved reliability was developed up to a prototype status, which can be transferred to industrial production

Enameled Glass Panels for Solar Thermal Building Envelopes

The paper presents a novel concept of solar thermal panel specifically intended for building integration, aiming at a higher architectural quality and at a reduction of installation costs. The panel consists of a low-emissivity enameled flat glass as solar absorber and a metallic heat exchanger, which are glued together by an adhesive layer. It features high design flexibility and can be used as roof or façade cladding in combination with common frames and profiles. We analyze the potential of the panel both as uncovered and covered collector by means of efficiency measurements on large-sized prototypes according to ISO 9806. Our results show that panels equipped with black enameled glass can achieve performance values competitive with those of commercial available products (uncovered panel: η0 = 0.75, b1 = 8.05 W/m2K, b2 = 1.64 J/m3K, bu = 0.043 s/m; covered panel: η0 = 0.74, a1 = 4.26 W/m2K, a2 = 0.013 W/m2K2). As reported by our optical measurements on small samples, colored glass can exhibit solar absorptance up to 0.93, thus representing an aesthetically appealing alternative to black panels. For its implementation, system integration, operating conditions and design aspects have to be taken into consideration

High transmittance, low emissivity glass covers for flat plate collectors: Applications and performance

Low emissivity coatings can significantly reduce radiative heat losses of glass panes for solar energy use. Their effectiveness is strongly dependent on their optical properties, which need to meet the requirements for the specific application. The paper analyses the performance of newly developed, highly transmitting and spectrally selective glass coatings based on transparent conductive oxides (TCO) for the use in flat plate collectors: Uncovered, single-glazed and double-glazed designs are taken into consideration. As functional layers both tin-doped indium oxide and aluminum-doped zinc oxide have been investigated. Collector efficiencies and annual collector yields are presented and compared to those of state-of-the-art collectors. Theoretical calculations are complemented by measurements on collector prototypes. The results show that a significant performance increase is accessible both in single-glazed collectors with low or non-selective absorbers and in double-glazed collectors with highly selective absorbers

Optimal Connection of Heat Pump and Solar Buffer Storage under Different Boundary Conditions

The paper presents the results of a simulation study, in which the connection of heat pumps and buffer storage tanks has been investigated. The simulations are carried out for a new type of a solar thermal combi system with a 32 m2 collector field leading to a solar fraction of more than 50%. In the first stage, the most influencing installation and operation parameters have been identified and optimized for typical boundary conditions of weather/climate, hot water demand, building and space heating system. Within further simulations these boundary conditions are varied to find generalized design rules for the connection of heat pumps and storage tanks. These results are presented and discussed

Pipe internal recirculation in storage connections - An unseen efficiency barrier

Basically, pipe internal recirculation (PIR) at storage connections is a known phenomenon. However, it is not sufficiently considered by storage manufacturers and installers. There is still a lack of awareness and quantification of the PIR effect in typical installations and practice-oriented solutions. Within this paper, the final results of a research project aiming at the detailed qualitative description and quantification of the effect including extrapolation of the results regarding the PIR impact on storage heat losses are presented. This includes the results regarding the influence of the pipe's material (copper, stainless steel, plastic), diameter, insulation and connection type (direct to storage, indirect via immersed or external heat exchanger) on PIR including their effects on the overall heat losses of ready installed storage tanks. Derived from generalized measurements, a calculation method has been developed, which enables to determine the annual heat losses caused by PIR in domestic hot water storages using dynamic system simulations (TRNSYS). In conclusion, different measures for the reduction of PIR induced heat losses and their effectiveness may be presented. © 2015 The Authors

Butane Heat Pipes for Stagnation Temperature Reduction of Solar Thermal Collectors

Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry out limit. Typically water, pentane or acetone are used as heat transfer media in collector heat pipes. Butane is very suitable to reach a high temperature gradient of the dry out even if the maximum temperature in the fluid circuit should be designed to 120 °C or below. The paper presents experimental results with butane heat pipes that operate up to a maximum temperature of 120 °C with a high temperature gradient in the dry-out region. This ensures that the collector performance in the operating range (typically up to 100 °C) is not affected negatively by the dry-out. Different approaches to increase the thermal conductance of butane heat pipes by enhancing the inner surface of the condenser or of both, the condenser and the evaporator are experimentally assessed and discussed. Measurement results report an increase of the heat pipes' thermal conductance from 3 W/K (standard geometry) to 23 W/K