General method for the evaluation of complex heat pump cycles

Whenever the fractional temperature lift ΔT/T_c of a heat pump is 0.15, simple cycles with one-stage throttling exhibit unsatisfactory energy performance. The adoption of multi-stage throttling, both in non-regenerative and regenerative cycles, is the most direct way of improving the cycle coefficient of performance (COP). The performance of these complex cycles is found to be a function of the molecular complexity of the working fluid, the reduced evaporation temperature, the fractional temperature lift and the number of stages of throttling. Furthermore, complex cycles are shown to be equivalent to a combination of simple cycles and their performance may be directly inferred by this route. Such calculations show that for a given fractional temperature lift an optimum molecular complexity (between that of R12 and n-butane) exists. Fluids with simpler molecules exhibit excessive vapour superheating during compression, and those with more complex molecules have excessive throttling losses. Also, with complex cycles, regeneration should be applied only to the cycle at the lowest temperature in order to improve the cycle COP and to prevent condensation during compression. As a general trend, however, complex cycles suffer a significant loss in performance compared to optimized simple cycles due to the adverse area of the two-phase diagram in which they work.     

Review article from ASHRAE symposium, San Francisco, January 1986: Transient performance and frosting of unitary heat pumps

Three papers relating to the transient performance and frosting of unitary air-source heat pumps were presented at this symposium. The titles and respective authors of the papers are:

1. 1. Effect of short cycling and fan delay on the efficiency of a modified residential heat pump by W.J. Mulroy, National Bureau of Standards.

2. 2. Air-to-air heat pumps operating under frosting conditions on the outside coil by C. Tantakitti and R. H. Howell, University of Missouri, Rolla.

3. 3. Design and preliminary analysis for measuring transient mass rate of flow in unitary heat pumps by M. I. Belth and D. R. Tree, Purdue University, West Lafayette.

A short review of each of these is given in this Paper.     

Compression heat pump with solution circuit Part 2: sensitivity analysis of construction and control parameters

In this Paper the results of a sensitivity analysis of a number of construction and control parameters of a compression heat pump with solution circuit (CHSC) are presented. The analysis is calculated with a computer model which is verified by the experimental results from a test plant.     

Compression heat pump with solution circuit Part 1: design and experimental results

This Paper presents the design and experimental results of a compression heat pump with solution circuit (CHSC) that has been constructed at the Swiss Federal Institute of Technology. The CHSC offers two major advantages over a single fluid Rankine cycle: 1, the heating capacity is easily varied by a large factor by adjusting the composition of the mixture; and 2, the approximation of the Lorenz process allows for substantially high COP values in cases with gliding temperatures. The test plant heats water from 40 to 70°C and cools water from 40 to 15°C. The heating capacity can be varied from 5 to 15 kW. A COP of 4.3 was measured, representing an energy saving of 23%, compared with a good single fluid Rankine cycle. Measured heat and mass transfer coefficients are presented and discussed.     

Fluids for high temperature heat pumps

The theoretical performances of some 250 potential work fluids in vapour compression heat pumps condensing at 150°C and evaporating at 100°C have been predicted, using expression for coefficient of performance (COP) and minimum superheat that involve only easily accessible physical properties. Expected correlations were found between COP and critical temperature, between specific compressor displacement and normal boiling point, T_bp, and between condensing pressure and T_bp. Correlations were also found between minimum superheat and both molecular weight and critical pressure. From these correlations, the desirable basic properties of a high temperature heat pump fluid are deduced. The principle of corresponding states is invoked to explain the connection between minimum superheat and critical pressure, and hence the reason why perfluorinated compounds tend to make poor work fluids.     

Reciprocating compressors for refrigeration and heat pump application

Reciprocating compressors have been used in refrigeration for about 150 years. At first they operated at a maximum of 100 r.p.m. and so were very large. Much of the development has been directed to increasing speed and hence reducing size and cost. Recent improvements have been directed mostly to valve design because valve performance largely controls the indicator diagrams and hence the isentropic efficiency. Present studies are directed to improving the mechanical efficiency. Despite all this progress, further improvement is still possible.     

Energy roof as heat pump source/sink

An energy roof was considered both as a source for a heat pump and as a sink during the summer for a machine acting as an air conditioner. Performance looks very promising compared with similar systems which are equipped with finned coils utilizing the outside air as source/sink.     

Heat pumps—where are improvements possible? An exercise in exergy

The true second law efficiency of presently available heat pumps is rather low and represents an impediment to their use, particularly in heat energy based countries. Energy analysis can serve to separate the individual losses and indicate ways to improve the process.     

Predicting heat pump performance by thermodynamic generalizations of fluid flow and heat transfer data

Thermodynamic generalizations based on reduced pressure proposed in the 1960s are reviewed and updated to reflect the current state of the art. The application of the method is illustrated by analytical and numerical examples and an assessment made of its value in heat exchanger design practice with special emphasis on two-phase forced convection refrigeration cycle applications. It is shown that this thermodynamic approach provides the heat exchanger designer, and to some extent the system engineer with an additional tool which is simple, effective and above all more reliable, particularly in evaporator and condenser design practice, than current conventional semi-empirical correlations.     

Thermo-economic analysis of refrigeration and heat pump systems

This Paper is based on a lecture presented the Austrian Society of Refrigeration Engineers. The optimization of refrigeration and heat pump systems discussed and a thermo-economic analysis is outlined using the Second Law of Thermodynamics.     

Calculation of the performance of vapour compression heat pumps with solution circuits using the mixture R22-DEGDME

In this paper the results of performance calculations of two vapour compression heat pump cycles, one with a single-stage solution circuit and one with a two-stage solution circuit, are presented. As a working fluid combination R22-DEGDME was selected out of a number of candidates in order to employ a safe and non-toxic mixture. It was found that both cycles show a significant increase in coefficient of performance (COP) (up to 50% for the two-stage cycle) compared to R22. The two-stage cycle shows a pressure ratio which is only 45% that for pure R22, resulting in increased mass flow rate but reduced capacity.     

Heat pumps in the USA

First commercially introduced in the 1930s, heat pumps are now used to heat, or heat and cool, approximately 30% of all new buildings, both residential and commercial, in the USA. Their use is considerably lower for domestic water and industrial process heating, and is still at the pioneering stage for district heating and cooling. Widespread installation of air conditioning, prevalent use of air rather than water for heat distribution, and generally lower energy prices result in major differences in heat pump acceptance compared to that in European countries. This Paper surveys the current status of heat pumps in the USA and factors influencing their future.     

The compression/absorption heat pump cycle—conceptual design improvements and comparisons with the compression cycle

Performance improvement of an industrial single-stage compression/absorption heat pump (CAHP) using an ammonia/water mixture as the working fluid has been studied theoretically. By allowing a higher absorber pressure (40 bar) than the highest design pressure of today's screw compressors (25 bar), higher COPs could be obtained. Longer falling-film tubes in the vertical shell-and-tube absorber and desorber also increased the COP. These two modifications together increased the COP of the CAHP by 10%. The improved design has a lower optimal absorber glide (temperature difference due to composition change in absorber) and reduced solution heat exchanger sizes. The study was performed with a constant total area. Furthermore, the CAHP performance was studied for five heating cases. Its performance was compared to that of a two-stage compression heat pump (CHP) using isobutane as working fluid, on the basis of approximately equal investment cost. It could be concluded that only heating cases where both the sink and the source temperature changes are high (>20 K) give superior performance for the CAHP.     

Compact heat exchangers, enhancement and heat pumps

In order to achieve widespread use of heat pumps across the full spectrum of potential applications, it is critical that the first cost of the units is acceptable. There are many factors influencing this cost, including the number of units manufactured, the ease of installation, the complexity of the control requirements, and the cost of the working fluid(s). A common feature of all heat pump cycles is the presence of at least one heat exchanger, indeed some heat-driven cycles are composed almost entirely of heat exchangers, each having a different but critical role to play. There are several important aspects of heat exchangers that can help to reduce first cost of these components and the system, (in addition to the possible positive impact on coefficient of performance). Two of these are discussed here — compact heat exchangers (CHEs) and heat transfer enhancement. The latter may be directly associated with CHEs but can be equally beneficial in reducing approach temperature differences in 'conventional' shell and tube heat exchangers. Both are essential features of many intensified processes, which the author argues need compatible heat pumps if the market for the latter is to flourish. In this paper, the most recent types of CHE are described, with emphasis on the benefits they can bring to heat pump first cost and performance. Heat transfer enhancement in heat pumps is also reviewed.     

Calculation of basic parameters for gravity-fed evaporators for refrigeration and heat pump systems

A method for calculating the basic parameters for gravity-fed evaporators has been developed based on the calculation of the pressure drop of two-phase flow of refrigerant over pipes and pipe components. Gravity-fed evaporators have a unique self-regulation ability and are among the most efficient and reliable refrigeration and heat pump systems, provided that they are correctly designed.     

Regional report Europe: “heat pumps — status and trends”

By 1997 about 90 million heat pumps have been installed worldwide, only less than 5% are located in Europe, historically the cradle of this “thermodynamic heating and cooling process”. The majority of the approximately 4 million installed heat pumps are imported reversible air-to-air systems in southern Europe and only 30% represent the typical European-made heating only electric driven compression systems for space and water heating in buildings in central and northern Europe. The first and second oil crises has been the main cause for a first European heat pump “boom” at the end of the seventies. Consequently the following drop in energy prices negatively influenced the market in some countries. The new renaissance in Europe in the middle of the nineties was initiated by the understanding of sustainable development for a more efficient energy use and the related protection of the environment.     

Heat sources — technology, economy and environment

Heat sources for heat pumps in buildings as well as in industry are discussed. Furthermore, some environmental aspects concerning choice of heat source are highlighted. Only systems for heat pumping are included, i.e. air-conditioning types which can also partly work as heat pumps are excluded. Recent heat pump installations in Sweden are mainly in small systems. Ambient air, exhaust air, soil and rock are the most common heat source types. Data on COP, investment costs, working fluids, present Swedish market etc. for these types of heat sources are given. Data on industrial heat pump installations in some countries and distribution of these according to heat pump type and industry sector as well as heat source temperatures are reported. Process integration aspects when choosing heat source size and temperature are also discussed as well as the relation between these parameters and the choice of heat pump type. Finally, the influence on the economy of the heat source temperature is presented. The cost-effectiveness of heat pumps for reducing CO₂ emissions compared with other heating technologies is discussed. The main results are: (1) heat pumps can in many cases in the future contribute to a reduction of CO₂; (2) there is a rather big difference, for larger systems a major difference, between the water-based and the ambient air-based heat pumps in terms of efficient reduction of greenhouse gas emissions.     

Thermophysical properties of the trifluoroethanol-pyrrolidone system for absorption heat transformers

Thermophysical properties of the 2,2,2-trifluoroethanol-2-pyrrolidone system have been investigated. The scope of the study includes density, viscosity, thermal conductivity, the equilibrium vapour pressure-concentration-temperature relationship, specific heat capacity, heat of mixing and enthalpy. Experimental measurements were carried out. Mathematical correlations for various properties were mainly derived from measured data. The good combination of physical and thermal properties of trifluoroethanol and pyrrolidone shows that they can be used as a working pair for absorption heat transformers.     

Effect of soil type and moisture content on ground heat pump performance: Effet du type et de l''humidité du sol sur la performance des pompes à chaleur à capteurs enterrés

Three different soils (sand, silty loam and silty clay) with five different degrees of saturation (0, 12.5, 25, 50 and 100%) were used in computer simulations. The performance of a ground heat pump system was found to depend strongly on the moisture content and the soil type (mineralogical composition). Alteration of soil moisture content from complete dryness to 12.5% of saturation strongly influences the ground heat pump performance, and any decrease of soil moisture in this range has a devastating effect on the coefficient of performance (COP). Therefore, it is beneficial to keep the soil moisture value as high as possible above dry soil conditions. Soil moisture content above the quarter saturation state leads to a much better heat pump performance. It was found, however, that the effect of moisture content variation above 50% of saturation on ground heat pump performance is relatively insignificant.