Developments in geothermal energy in Mexico—part thirty-six. The commissioning of a heat pump assisted brine purification system

Any system which extracts useful energy from a geothermal source is limited by the effectiveness of the heat transfer between the geothermal fluid, which has a tendency to scale, and the relevant components of the system. A heat pump can be used to maintain a temperature difference between two vessels containing pure water and geothermal brine which is sufficient to enable pure water vapour to flow continuously from the geothermal brine vessel to the pure water vessel. A mechanical vapour compression heat pump using R114 as the working fluid and designed to deliver more than 50 kW of heat has been incorporated in a brine purification system at the Los Azufres geothermal field. This system has been successfully commissioned and is operating as designed and theoretically predicted.     

Theoretical studies on adsorption heat transformer using zeolite-water vapour pair

An adsorption heat transformer can raise the temperature level of a fraction of waste heat by rejecting the remaining heat to a low temperature level. In this work some alternatives in the design of an adsorption heat transformer, such as a 2-tank system, 3-tank system and 4-tank system, are evaluated using zeolite-water vapour as the adsorbent-adsorbate pair. The values of coefficient of performance (COP) are computed for each system for various temperatures of waste heat source at which the heat is available and heat sink at which the heat is delivered.It is found that an adsorption heat transformer can be used for a gross temperature lift as high as 50°C with a fairly good COP value. Moreover the 4-tank system gives a much improved COP value as compared to the 2-tank and 3-tank systems for the same operating conditions. It is also found that the effect of temperature driving force for heat transfer on the COP value is quite pronounced.     

Developments in geothermal energy in Mexico—part sixteen. The potential for heat pump technology

Mexico possesses large amounts of geothermal brine at temperature which are too low to enable electricity to be generated efficiently and economically. Any system which extracts useful energy from a geothermal source is limited by the effectiveness of the heat transfer between the geothermal fluid, which has a tendency to scale, and the relevant components of the system. A heat pump can be used to maintain a temperature difference between two vessels containing pure water and geothermal brine, which is sufficient to enable pure water vapour to flow continuously from the geothermal brine vessel to the pure water vessel. The hot water produced can then be used to operate an absorption cooling system which can be used to store food. Alternatively a heat pump can be employed to increase the temperature of the hot water to produce low pressure stream.     

Analysis of an integrated vapour compression and a waste heat dehumidifier A/C system

A compact, self-contained energy efficient air-conditioning system is presented. It has a vapour compression (VC) unit combined with a rotary solid desiccant dehumidifier. The drying matrix is regenerated entirely using the waste heat of the VC unit by a heat pump in a heat recovery system.The governing equations are given and are numerically solved for a typical case study. Computations show that employing parallel flow rather than counterflow configurations through the matrix can reduce the regeneration heat required by about 40%. Equations are solved for a practical range of cycle duration from 180 to 900 s. Results show the variations of temperature and water content for the matrix as well as the flowing air streams.The overall cooling COP achieved by the combined system reached 1.73 for design conditions in Jeddah, Saudi Arabia, which is 25% more than that of the VC alone. Whenever the ARI conditions are applied, the combined system showed an overall COP of 1.76 with 27% energy savings compared to the VC alone.     

Winter performance of residential heat pump

This paper deals with the effect of a snowcover on the coefficient of performance (COP) of a residential heat pump with a single layer ground heat exchanger for maritime climatic conditions (Fredericton, NB, Canada). A computer program was used to simulate a snowcover and heat and moisture transfer in the ground. Results obtained show that an increased thickness of a snowcover may evoke an increase of the COP of the ground heat pump system. Noticeable changes of the COP are observed within a snow-depth range from 0 to 10 cm. A further increase in snow depth does not significantly improve the COP which hereafter appears to be independent on a depth of a horizontal ground heat exchanger.     

Developments in geothermal energy in Mexico—part thirty-nine. Extraction of useful heat and silica removal from geothermal brine utilizing liquid fluidized bed heat exchangers

Liquid fluidized bed heat exchangers can be utilized for the recovery of heat from geothermal brine in order to avoid scaling. At the same time, these heat exchangers remove silica from the geothermal brine. A new parameter, obtained from the bed expansion and referred to as the porosity of transition, is defined. This parameter contributes to a better understanding of systems utilizing liquid fluidized bed heat exchangers. From the model proposed by Goto [J. Phys. Chem.60, 1007–1008 (1956)] for silica polymerization and modified by Axtmann [Geothermics15, 185–191 (1986)] for silica removal, the rate of reaction constant is expressed as a function of silica equilibrium monomer concentration (solubility).     

Thermodynamic analysis of R22-DMF compression-absorption heat pump

The performance of the compression-absorption heat pump working on R22-Dimethyl formamide (DMF) is studied through a thermodynamic analysis. The parameters varied are the compression ratio and the operating temperatures at the desorber and absorber. The heating coefficient of performance, exergetic efficiency, concentration difference and the circulation ratio are computed. At certain operating conditions COPs as high as 6 and temperature lifts as high as 60°C can be achieved. Correlations are presented to facilitate quick estimation of performance chatacteristics.     

Effect of operating temperatures on the coefficient of performance of active carbon-methanol systems

Experimental and predicted results of an active carbon (AC35)-methanol pair as a function of operating temperatures are discussed.Experimental COP's higher than 0.5 are obtained when the evaporating temperature lift is less than 25°C. The COP is seen to be very sensitive to the evaporating and adsorbing temperatures. The regenerating temperature is of the order of 100°C which makes that pair a good candidate for solar cooling applications.A comparison between the results obtained with that pair and predicted performances of three other pairs (NH₃H₂O, ZeoliteH₂O and another active carbon-methanol pair) shows that:

• 1.(1) two adsorber cycles give higher COP's than the NH₃H₂H₂O continuous cycle;
• 2.(2) intermittent adsorptive cycles give higher COP's than the NH₃H₂O continuous cycle as long as the evaporating temperature lift is less than 50°C;
• 3.(3) among the solid adsorbents, the active carbon-methanol pairs seem to be the most promising for cooling applications. The choice of the active carbon depends on the evaporating temperature lift: AC35 is very well adapted for temperature lifts larger than 30°C; ACLH could be better adapted to smaller evaporating temperature lifts, as suggested by Passos et al.

     

Psychometric technoeconomic assessment and parametric studies of vapor-compression and solid/liquid desiccant hybrid solar space conditioning systems

This communication presents some studies on hybrid vapor-compression (V-C) and solid/liquid desiccant solar air-conditioning systems. Desiccants are used to bring the humidity within the comfort range by partially converting the latent heat load (LHL) into sensible heat load and then meeting this load with conventional V-C cycle. In both the options of hybrid systems consisting of vapor-compression and solid/liquid desiccant cycles, heat rejected from the condenser of the V-C cycle is used to regenerate the desiccants to improve the coefficient of performance (COP) of the systems. A thermodynamic assessment and technoeconomic feasibility study of hybrid V-C and solid/liquid desiccant solar air-conditioning systems have been presented. It is found that a hybrid system with desiccant cycle is more promising under high latent heat load and higher ambient humidity conditions and significant energy saving can be achieved over standard V-C cooling systems.     

Performance characteristics and parametric optimization of an irreversible magnetic Ericsson heat-pump

Taking into account the finite-rate heat transfer in the heat-transfer processes, heat leak between the two external heat reservoirs, regenerative loss, regeneration time, and internal irreversibility due to dissipation of the cycle working substance, an irreversible magnetic Ericsson heat-pump cycle is presented. On the basis of the thermodynamic properties of magnetic materials, the performance characteristics of the irreversible magnetic Ericsson heat-pump are investigated and the relationship between the optimal heating load and the coefficient of performance (COP) is derived. Moreover, the maximum heating load and the corresponding COP as well as the maximum COP and the corresponding heating load are obtained. Furthermore, the other optimal performance characteristics are discussed in detail. The results obtained here may provide some new information for the optimal parameter design and the development of real magnetic Ericsson heat-pumps.     

Conceptual thermodynamic design of a dual mode absorption cycle with an auxiliary heat exchanger

This paper presents a conceptual thermodynamic design and feasibility study of a dual mode absorption cycle with auxiliary heat exchanger. The system consists of two single stage absorption cycles connected through the condenser of the first stage with the generator of the second stage. The purpose of the auxiliary heat exchanger is to maintain the condenser (1) temperature sufficiently high to operate the generator (2). A case study, using a single fluid pair H₂OLiBr in both the stages and two fluid pairs (H₂OLiBr at first stage and NH₃LINO₃ at second stage) has been considered. It is found that the COP for the two pair system in comparison to the single pair system. However, the former has the advantage of providing simultaneous refrigeration and air-conditioning.     

Comparison of mixture and pure refrigerant heat pump cycles

The use of refrigerant mixture has been shown, by some researchers, to exhibit higher COPs than pure refrigerants in vapor compression cycles. However, in order to compare the cycles fairly, heat exchanger size should be taken into consideration. The work contained herein compares the use of a mixture of R22/R114 and pure R22 in a heat pump operating in the heating mode. The thrust of the effort was to present the comparison on the basis of equal heat exchanger size.     

Thermodynamic analysis data on vapour absorption heat pump working on R22-DMETEG pair

A thermodynamic analysis of the vapour absorption heat pump working on R22-DMETEG pair is made and the results are presented in the form of graphs and correlations. These data can be used to estimate the idealised system performance parameters such as heat quantities, flow rates, pump work, COP and second law efficiency. The procedure for using the data for above computations is described.     

Estimation of the required collector area of a solar assisted heat pump

The solar assisted heat pump represents a method of increasing the heat pump COP by transferring solar radiation heat to the heat pump evaporator. In this paper a method of calculating the minimum design limit for the collector area based on the second law efficiency is introduced by using a set of experimental data on a certain design of a solar assisted heat pump system during winter time. A comparative study is made between the actual and ideal conditions, and a minimum required area of the collector is suggested.     

Thermodynamic design and assessment of hybrid double absorption solar cooling systems

This communication presents an investigation of the thermodynamic design and feasibility assessment of hybrid double-absorption solar cooling systems. The systems considered are conventional closed-cycle and open-cycle absorption systems with an additional open-absorber component through which the process room air is passed, cooled and dehumidified. The cooling produced in the evaporator is utilized to remove heat from the open absorber as well as the process air being circulated. Thermodynamic modelling of system components for a fixed set of operating conditions and with water-LiBr as working fluid have been carried out and the system COP predicted as a function of regenerator temperature, mass flow ratio and latent heat load in the process air and the ambient air. A comparative study of the open and closed cycle options has been made and detailed parametric results are presented. It is concluded that the hybrid double-absorption solar cooling systems are better in performance than conventional systems and an open-cycle double-absorption system is even more attractive and cost effective as compared to closed-cycle option.     

Thermodynamic study of advanced absorption heat transformers—I. Single and two stage configurations with heat exchangers

A thermodynamic analysis was carried out to study the effect of heat exchanger effectiveness (EF) on the performance of single stage heat transformers (SSHT). Moreover, an analysis of three different arrangements of two stage heat transformers was performed using a mathematical model assuming water/lithium bromide as the working fluid. An increase in the solution heat exchanger effectiveness (EF) greatly improved the performance of absorption heat transformers when the absorber temperature was at least 40°C higher than the temperature of the heat supplied to the system. In two stage heat transformers (TSHT), higher absorber temperatures were obtained by coupling the absorber of the first stage to the evaporator of the second. However, higher performance coefficients were obtained in general by coupling the absorber of the first stage to the generator of the second.     

Thermodynamic design data for double effect absorption heat pump systems using water-lithium chloride—cooling

In this paper, thermodynamic design data are investigated for the water-lithium chloride pair and a comparative study of the water-LiCl pair with the water-LiBr pair is given, for double-effect absorption cooling systems used for the computer simulation. The computer simulation is based on mass, material and heat balance equations for each part of the system. Coefficients of performance and flow ratios for effects of different operating temperatures are investigated. It is found that the cooling COP is higher for the water-LiCl pair than for the water-LiBr pair, and FR is lower for the water-LiCl pair than for the water-LiBr pair.     

Simulation and thermodynamic design data study on double-effect absorption cooling cycle using water-LiBr-LiSCN mixture

In this study, for double-effect absorption cooling systems, a computer simulation which is based on mass, material and heat balance equations for each part of the system is described. Thermodynamic design data are investigated for the water-lithium bromide-lithium thiocyanate mixture and a comparative study of the water-LiBr-LiSCN mixture with the water-LiBr pair and the water-LiCl pair is given. The influences of component temperatures on the cooling coefficients of performance have been investigated to obtain operating condition. It is found that the cooling COP is higher for water-LiBr-LiSCN mixture than for the water-LiBr pair or for the water-LiCl pair, and the flow ratio of water-LiBr-LiSCN mixture is not higher than that of the water-LiBr pair.     

Investigation of the performance of a heat pump under frosting and defrosting conditions

This paper investigates the effects of frosting and defrosting the evaporator coil on the performance of domestic size air-to-water heat pump systems. The results show that the degradation in the COP caused by frosting and defrosting is dependent upon the ambient conditions and the duration of the frosting period. Optimum performance can be obtained by implementing defrost control strategies based on real time performance data on a microprocessor based control system.