Analysis of utilisation of renewable energies as heat sources for heat pumps in building sector in Poland

In this paper the emphasis is put on identification the problem how different renewable sources can be effectively used as heat sources for heat pumps in Poland. Main data of: ambient air, solar radiation, soil, ground and surface water are presented. Recommendations for heating systems with a heat pump are given.     

Performance and economic feasibility of ground source heat pumps in cold climate

Ground source heat pumps (GSHP) are attractive alternatives to conventional heating and cooling systems owing to their higher energy utilization efficiency. In this paper, the effect of various system parameters on GSHP performance is studied using a computer model. Also, a comparative economic evaluation is carried out to assess the feasibility of using a GSHP in place of conventional heating/cooling systems and an air source heat pump. The results indicate that system parameters can have a significant effect on performance, and that GSHP is economically preferable to conventional systems. © 1997 John Wiley & Sons, Ltd.     

Vertical-borehole ground-coupled heat pumps: A review of models and systems

A large number of ground-coupled heat pump (GCHP) systems have been used in residential and commercial buildings throughout the world due to the attractive advantages of high efficiency and environmental friendliness. This paper gives a detailed literature review of the research and developments of the vertical-borehole GCHP technology for applications in air-conditioning. A general introduction on the ground source heat pump system and its development is briefly presented first. Then, the most typical simulation models of the vertical ground heat exchangers currently available are summarized in detail including the heat transfer processes outside and inside the boreholes. The various design/simulation programs for vertical GCHP systems primarily based on the typical simulation models are also reviewed in this paper. Finally, the various hybrid GCHP systems for cooling or heating-dominated buildings are well described. It is found that the GCHP technology can be used both in cold and hot weather areas and the energy saving potential is significant.     

Efficiency of closed loop geothermal heat pumps: A sensitivity analysis

Geothermal heat pumps are becoming more and more popular as the price of fossil fuels is increasing and a strong reduction of anthropogenic CO₂ emissions is needed. The energy performances of these plants are closely related to the thermal and hydrogeological properties of the soil, but a proper design and installation also plays a crucial role. A set of flow and heat transport simulations has been run to evaluate the impact of different parameters on the operation of a GSHP. It is demonstrated that the BHE length is the most influential factor, that the heat carrier fluid also plays a fundamental role, and that further improvements can be obtained by using pipe spacers and highly conductive grouts. On the other hand, if the physical properties of the soil are not surveyed properly, they represent a strong factor of uncertainty when modelling the operation of these plants. The thermal conductivity of the soil has a prevailing importance and should be determined with in-situ tests (TRT), rather than assigning values from literature. When groundwater flow is present, the advection should also be considered, due to its positive effect on the performances of BHEs; by contrast, as little is currently known about thermal dispersion, relying on this transport mechanism can lead to an excessively optimistic design.     

Heat transfer in ground heat exchangers with groundwater advection

In order to estimate the impact of groundwater flow on performance of geothermal heat exchangers in ground source heat pump systems, an equation of conduction–advection is established for heat transfer in porous media, and an analytical transient solution is obtained for a line heat source in an infinite medium by means of the Green function analysis. An explicit expression has also been derived of the mean temperature on circles around the heat source. Dimensionless criteria that dictate the process are summarized, and influence of the groundwater advection on the heat transfer is discussed accordingly. Computations show that water advection in the porous medium may alter significantly the conductive temperature distribution, result in lower temperature rises and lead to a steady condition eventually. The hydraulic and thermal properties of soils and rocks influencing the advection heat transfer are briefly summarized. The analytical solution has provided a theoretical basis and practical tool for design and performance simulation of the ground heat exchangers.     

The geothermal characteristics of the ground and the potential of using ground coupled heat pumps in Cyprus

The ground can be used as an energy source, an energy sink, or for energy storage and for this reason ground characteristics should be available. Therefore the purpose of this paper is to present the recorded ground temperatures at eight representative sites of Cyprus, in relation to depth, time of year, geology and altitude and discuss the efficiency of Ground Coupled Heat Pumps. The ground temperature was recorded for a period of one year, from October 2009 to October 2010. According to the results obtained, in several locations in Cyprus the surface zone reaches a depth of 0.5 m. The shallow zone penetrates to 7-8 m and there after the deep zone follows in which the temperature remains constant throughout the year with a range between 18 and 23 °C. For the eight boreholes, additional geothermal data were also recorded like the type of ground and thermal conductivities of the various geologic layers. The data collected clearly indicate that there is a potential for the efficient use of Ground Coupled Heat Pumps (GCHPs) in Cyprus leading to significant savings in heating and cooling energy consumption.     

地源热泵的分类及其若干问题的看法

针对地源热泵的分类和名称不很统一的现状，提出了按低温热源的来源统一分类的看法，并据此对目前常用的各种土壤源、水源热泵进一步分类；并提出目前把水环热泵当作水源热泵是不合适的，应该作为空气源热泵的一种。     

Limiting performance of vapour compression heat pumps for heating fluids

When a vapour compression heat pump is used to heat a fluid over a range of temperatures the refrigerant cycle may be fundamentally different from a standard Rankine cycle which operates only between isothermal reservoirs. This paper considers two strategies for heating fluids: in one approach the fluid is heated in a single pass condenser; in the other the body of fluid is kept fully mixed at all stages of the heating process. An isothermal source reservoir is assumed and the performance of nine common refrigerants is examined. In the ideal limit the mixed fluid process has a higher COP than the single pass method. But in practice there is likely to be no clear advantage in using a fully mixed sink fluid, especially where the temperature lift is greater than 50°C. The single pass heating process has an advantage under these conditions that lower peak condensing temperatures may generally be achieved than for the mixed fluid heating process.     

Improved underground heat exchanger by using no-dig method for space heating and cooling

This paper describes experiments and analyses on an improved underground heat exchanger by using a no-dig method for the purpose of the cost reduction of a space heating and cooling system using underground thermal energy. First, the improved underground heat exchanger was installed on the campus of Hokkaido University, and it was shown that a ground source heat pump system utilizing the heat exchanger was sufficient for space heating and cooling. Second, evaluation program of the heat exchanger was developed, and the program was verified to give good predictions by comparing with experimental results. As a result of system simulations, an energy reduction for a system installation relative to a conventional vertical earth heat exchanger reached 78%. The primary energy reduction rate including the system installation and operation relative to a typical air source heat pump was 29%.     

土壤源热泵的研究与开发

土壤源热泵是利用地下土壤能源资源来进行供暖空调的一种高效、节能、环保型空调技术，近年来得到了快速的发展；文章介绍了它的国内外研究状况，分析了土壤的传热特性、土壤的温度分布状况及埋地盘管的传热特性，建立了埋地盘管的传热数学模型，指出了土壤源热泵研究与发展中的关键性问题，最后展望了其应用前景。     

Numerical simulation and performance assessment of a low capacity solar assisted absorption heat pump coupled with a sub-floor system

A prototype low capacity (10 kW) single stage Li–Br absorption heat pump (AHP), suitable for residential and small building applications has been developed as a collaborative result between various European research institutes and industries. The primary heat source for the AHP is supplied from flat plate solar collectors and the hot/chilled water from the unit is delivered to a floor heating/cooling system. In this paper we present the simulation results and an overview of the performance assessment of the complete system. The calculations were performed for two building types (high and low thermal mass), three climatic conditions, with different types of solar collectors and hot water storage tank sizes and different control systems for the operation of the installation. The simulations were performed using the thermal simulation code TRNSYS. The estimated energy savings against a conventional cooling system using a compression type heat pump was found to be in the range of 20–27%.     

Transient 3D analysis of borehole heat exchanger modeling

This paper presents the development and application of a three-dimensional (3D) numerical simulation model for U-tube borehole heat exchangers (BHEs). The proposed model includes the thermal capacities of the borehole components, viz., the fluid inside the tubes, as well as the grouting material, making it possible to consider the transient effects of heat and mass transports inside the borehole. In this approach, the use of simplified thermal resistance and capacity models (TRCMs) provides accurate results while substantially reducing the number of nodes and the computation time compared with fully discretized computations such as finite element (FE) models. The model is compared with a fully discretized FE model which serves as a reference. Furthermore, the model is used to evaluate thermal response test (TRT) data by the parameter estimation technique. Comparison of the model results with the results of an analytical model based on the line-source theory further establishes the advantage of the developed 3D transient model, as the test duration can be shortened and results are more accurate.     

Potential of ground heat source systems

This paper presents a historical background of ground source heat pump technology, followed by a review of its current shortcomings. Based on these observations the author assesses the R&D needs and recommendations for future research.     

Heat transfer analysis of boreholes in vertical ground heat exchangers

A ground heat exchanger (GHE) is devised for extraction or injection of thermal energy from/into the ground. Bearing strong impact on GHE performance, the borehole thermal resistance is defined by the thermal properties of the construction materials and the arrangement of flow channels of the GHEs. Taking the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs into account, a new quasi-three-dimensional model for vertical GHEs is established in this paper, which provides a better understanding of the heat transfer processes in the GHEs. Analytical solutions of the fluid temperature profiles along the borehole depth have been obtained. On this basis analytical expressions of the borehole resistance have been derived for different configurations of single and double U-tube boreholes. Then, different borehole configurations and flow circuit arrangements are assessed in regard to their borehole resistance. Calculations show that the double U-tubes boreholes are superior to those of the single U-tube with reduction in borehole resistance of 30-90%. And double U-tubes in parallel demonstrate better performance than those in series.     

Ground source heat pump––description and preliminary results of the Eco House system

A reversible ground source heat pump has been installed and tested at the Eco House, University of Nottingham. Results obtained during experimental testing are presented and discussed here. Various loop designs were considered including a tank coil system that aids the heat transfer process via rainwater drainage. The results show that the design coefficient of performance (COP) of a heat pump unit can be achieved through proper installation and careful consideration of the operation of the system as a whole.     

Performance of ejector heat pumps

An ejector-compression heat pump can use low-grade thermal energy in the neighbourhood of 93.3°C (200°F) to provide space cooling and heating. This paper applies the existing ejector theory to estimate the performance of an ejector heat pump system at various operating conditions. The study includes parametric, sensitivity and off-design analyses of the heat pump performance. The performance enhancement options and desired ejector geometry are also examined. Refrigerants 11, 113 and 114 are three of the halocarbons most suitable for the ejector heat pump system. The estimated coefficients of performance for a simple ejector heat pump are 0.3 for the cooling mode and 1.3 for the heating mode at a sample operating condition in which the refrigerant (R-11) boiling temperature is 93.3°C (200°F), condensing temperature 43.3°C (110°F) and evaporating temperature 10°C (50°F). A 24 per cent performance improvement is predicted for a heat pump with two-stage ejectors and regenerative heat exchangers. The off-design performance is relatively insensitive to the evaporator temperature variations.     

江水源热泵在世博场馆中的应用

文章简要介绍了江水源热泵在世博场馆中的应用情况,并分析讨论了江水源热泵中的关键技术问题,对系统可靠性的保障和能源效率的提高提出了建议。     

Very low temperature radiant heating/cooling indoor end system for efficient use of renewable energies

Solar or solar-assisted space heating systems are becoming more and more popular. The solar energy utilization efficiency is high when the collector is coupled with indoor radiant heating suppliers, since in principle, lower supply temperature means lower demand temperature and then the system heat loss is less. A new type radiant end system is put forward for even lower supply temperature compared to the conventional radiant floor heating systems. A three dimensional model was established to investigate its energy supply capacities. Simulation results show that 50 W per meter length tube can be achieved with the medium temperature of 30 °C for heating and 15 °C for cooling. The predicted results agree well with the actual data from a demonstration building. Furthermore, it is demonstrated that a supply temperature of 22 °C in winter and of 17 °C in summer already met the indoor requirements. The new end system has good prospects for effective use of local renewable resources.     

Temperature variations near pipes used as a soil source for heat pumps

Calculations have been made to determine the variation with time of the temperature distribution around pipes buried 15 m below the soil surface, assuming the latter to be held at a constant temperature. For a fixed rate of heat extraction it is found that the soil temperatures reach quasi-static conditions in, at most, a few days after extraction commences, implying that simpler theories for steady-state temperatures are applicable. Calculations on the effects of grouping pipes together indicate that the pipes can be considered to be effectively isolated from one another if their separation is not less than 1 m.