Improvements in controlled-environment facilities for testing domestic air-to-water heat pumps

A control system is described, based on a microcomputer, which maintains constant temperature and relative humidity in a controlled-environment laboratory used for research on air-to-water heat pumps. The performance of the control system is measured under both steady-state and changing conditions, and optimum values of the control constants are deduced. The temperature can be maintained at the set value to within 0–1°C, and the relative humidity to within 1 per cent, under steady-state conditions. The effects of transients, such as the starting or stopping of the heat pump under test, or the deliberate changing of the controlled conditions, are studied in detail. In addition, a system is described for controlling the temperature of the condenser cooling water, which can be held to within 0–1°C of the set value.     

Measurement of the performance of domestic air-to-air heat pumps

It is desirable to test heat pumps over the whole range of conditions under which they will be used. A test facility built at BRE is described in which these conditions may be readily adjusted in a controlled manner, the test procedure for a heat pump unit being carried out in a relatively short time. the heat pump itself is utilized to attain conditions of lower temperature and humidity than ambient. The problems of measuring heat transport in air streams are outlined and ways of overcoming them are given. Particular attention should be given to mixing the air streams. Various aspects of instrumentation of such a test facility are discussed.     

Optimising heat exchangers for air-to-air space-heating heat pumps in the United Kingdom

The paper deals with some of the major aspects of heat-exchanger design for electric heat pumps. After a discussion of heat-transfer theory, it describes a method that can be used in the design and sizing of air-to-refrigerant heat exchangers and in calculating temperature distributions. As an illustration, economically optimum sizes for exchanger coils are given for heat pumps of output 13.9 kW, 5.6 kW and 5 kW at 5°C outside the ambient temperature. At several stages, manufacturer's experimental data have been used, and the final results are compared with the design of heat exchangers used in commercially available models. Some temperature measurements made on a heat-pump installation in an experimental house are also reported. At least doubling the size of presently used indoor coils is shown to be economically justifiable, increasing the seasonal coefficient of performance from about 2.4 to 2.8-3.0. Reassessment of outdoor-fan size is also shown to be necessary. Throughout the work it is assumed that the heat pump is required for heating only, as would be the case in the United Kingdom.     

Economic evaluation of heat pumps

An economic evaluation of heat pumps is presented which compares the running costs and discounted capital costs of central-heating systems based on oil-fired boilers and heat pumps. Conclusions regarding the break-even coefficients of performance are drawn for a range of fuel-price possibilities.     

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.     

The performance of heat pumps in service—A simulation study

A computer model is developed to simulate the performance of air-to-water heat pumps heating a house via a radiator system. The performance characteristics of the heat pumps are derived from laboratory measurements. Hourly weather observations are used to calculate the heat demand of the house and the performance of the heat pump. The effects of changes in heat pump characteristics, changes of radiator size, and changes in heat demand of the house due to insulation, are compared in terms of their effects on annual energy consumption.     

Steady-state simulation of vapour-compression heat pumps

A vapour compression simulation model was developed. Simple mathematical models were employed for each component of the cycle. They resulted in a set of nonlinear equations, which was solved numerically. Heat losses from condenser to ambient were included. The model is capable of predicting the operating point of the system (including condensing and evaporating pressures) as a function of equipment characteristics (for example, compressor swept volume, speed and clearance ratio, and heat exchanger overall conductances) and prevailing thermodynamic conditions (such as heat source and heat sink temperatures with the mass flow rates of their fluids). The predicted performance was compared to that of an existing R-12 unit, showing good agreement. As an application, a comparative analysis is made on the thermodynamic performance of a domestic heat pump running on two different refrigerants: R-12 and R-134a.     

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.     

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.     

The potential for heat pumps in drying and dehumidification systems I: Theoretical considerations

Drying is one of the most energy intensive unit operations. In many applications the drying temperatures required are low enough to make the inclusion of a heat pump in the system worthy of consideration. Five drying/dehumidification systems, including three with heat pumps, have been compared theoretically on the basis of specific power consumption (SPC), (i.e. the energy supplied per unit of moisture condensed) and primary energy consumption (PEC) which is (SPC) divided by the efficiency of primary energy conversion. The efficiency of each system is improved as the relative humidity of the air leaving the dryer is increased. The optimum is, however, very flat and a heat pump should be advantageous when a minimum relative humidity of 30 per cent is acceptable within the drying chamber. A closed cycle dryer is shown to be the most advantageous but requires careful matching.     

The potential for heat pumps in drying and dehumidification systems II: An experimental assessment of the heat pump characteristics of a heat pump dehumidification system using R114

An experimental heat pump dehumidifier is described. Actual coefficients of performance (COP)_A are plotted against the gross temperature lift (T_CO - T_EV) for various bypass ratios and air velocities. Interpolated values of (COP)_A for a specified temperature lift were obtained by fitting each set for various dry bulb temperatures of air leaving the humidifier using a linear equation. These values of (COP)_A are plotted against the linear velocity of the air stream approaching the evaporator at different dry bulb temperatures. The curves show a maximum of (COP)_A at approach velocities in the region of 1·6 ms^?1.     

Digital control of heat pumps with minimized power consumption

The paper describes the development of a microcomputer based control system for a heat pump containing an electrical variable speed compressor drive and a motorized expansion valve. It is designed to operate under very much varying load conditions with minimum power consumption. Difficulties that were encountered during engineering tests could finally be overcome by a relatively simple, practical regulator configuration. It operates near optimum efficiency by regulating a temperature difference in the evaporator.     

Use of controlled water circulation in tap water heat pumps

Condensers used in tap water heat pumps are either of the integrated passive immersion type or separate units with active water circulation. The use of the active circulation configuration together with a counter-flow condenser and water flow control has a number of advantages compared to passive immersion condensers. In particular, hot water temperatures above the saturation temperature of the condensing refrigerant are possible. The counter-flow arrangement also permits sub-cooling gains in cycle efficiency. These two features offset the pumping power penalty which would not be necessary in a system with a passive immersion condenser.     

The performance of heat pumps in service-comparison of a computer model with a real installation

A comparison is made between a computer model which was developed in an earlier paper and experimental results in which a heat pump was used to heat a house during the heating season 1978–1979. The heat pump used was an air-to-water machine, and it is found that the radiator temperature in the experiment varies according to the heat demand of the house because of the effect of thermal inertia of the water and other thermal masses in the heat transfer system. The computer model simulates this effect, using hourly weather data to calculate the heat demand of the house and assuming that the radiators run at the temperature necessary to supply the heat demand during each hour. The model also calculates the coefficient of performance of the heat pump, and hence calculates the running cost in kilowatt-hours for each hour. The calculated running cost is compared with daily readings of kilowatt-hour meters. It is found that the comparison is very accurate during normal operation of the heat pump, with an accuracy of better than 1 per cent over a period of four months of the heating season, although the accuracy is not always quite so good. A comparison is also made between hourly calculated radiator temperatures and continuous recordings of flow and return temperatures. The comparison in this case is satisfactory, but there is a time lag due to the effect of thermal inertia of the building fabric which the computer model is not intended to simulate.     

Assessment of a domestic heat pump 1: The house/heat-pump interaction

The operating environment for a domestic air-air heat pump installation is examined using data from a field monitoring study. The unit, which is used for heating only, has a seasonal COP of 2.7 ± 0.2. The system experiences a wide range of return air temperatures, which are also low compared with those of normal designs. We show that this is partly caused by the heating patterns adopted by the occupants, by which the primary living area is well heated but the secondary area receives background heating only.     

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.     

Evaluation of a microcomputer-based control system for a domestic sized engine-driven water-to-water heat pump

Steady-state performance data have been obtained on a domestic sized engine-driven water-to-water heat pump. The optimum working fluid suction superheat for the system was found to be 12°C. Over a range of heat sink conditions, increasing the engine speed linearly increased the total heat prouduced by the unit. Similarly, over a range of heat source conditions, increasing the engine speed linearly increased the working fluid evaporation rate. To produce water at 80°C, the heat pump was designed to operate with a heat sink temperature of 70°C, but its efficiency was improved by operating with the heat sink at 55°C. With a heat sink temperature of 55°C the primary energy ratio of the unit was observed to vary from 0–85 to 1–16, over a range of heat source temperatures. Algorithms developed from the steady-state experiments were incorporated as control function subroutines in a microcomputer program. Using this program, the microcomputer was employed to control the heat pump outlet water temperature and the working fluid suction superheat. The control system was tested in a series of dynamic experiments and was found to operate effectively and achieved its control requirements. In certain tests, the transient time period was extended because the electrically-controlled expansion valve was too large for the system and created instability in the suction superheat.     

Theoretical analysis of the dynamic interactions of vapor compression heat pumps

A detailed mathematical model of vapor compression heat pumps is described. Model derivations of the various heat pump components are given. The component models include the condenser, evaporator, accumulator, expansion device, and compressor. Details of the modeling techniques are presented, as is the solution methodology. Preliminary simulation results are also illustrated. The model developed predicts the spatial values of temperature and enthalpy as functions of time for the two heat exchangers. The temperatures and enthalpies in the accumulator, compressor and expansion device are modeled in lumped-parameter fashion. Pressure responses are determined by using continuity satisfying models for both the condenser and evaporator. The discussion of the solution methodology describes the combined implicit/explicit integration formulation that is used to solve the governing equations. The summary provides a list of future work anticipated in the area of dynamic heat pump modeling.     

Environomic multi-objective optimisation of a district heating network considering centralized and decentralized heat pumps

Concern for the environment has been steadily growing in recent years, and it is becoming more common to include environmental impact and pollution costs in the design problem along with construction, investment and operating costs.