Lithium bromide high-temperature absorption heat pump: Coefficient of performance and exergetic efficiency

A theoretical study on the employment of a lithium bromide absorption heat pump in Spain, used as machine type I and aimed to produce heat at 120°C via waste heat sources at 60°C, is given in the paper. Real performance conditions are stated for each component of the machine, namely the absorber, the heat recoverer, the generator, the condenser, the solution pump, the expansion valve and the evaporator. By means of thermodynamic diagrams (p, t, x) and (h, x), the required data are obtained for calculation of the heat recovered in the evaporator Q_e, and the heat delivered to the absorber Q_a and to the condenser Q_c, as well as the heat supplied to the generator Q_g. In addition, the heat delivered by the hot solution to the cold solution in the heat recovered Q_r, and the work W_p done by the solution pump are calculated. The probable COP is calculated, and values are obtained close to 1.4. The working temperature in the generator is determined; it ranges from 178 to 200°C. The heat produced by the lithium bromide absorption heat pump is 22% cheaper than the heat obtained from a cogeneration system comprising natural gas internal combustion engine and a high temperature heat pump with mechanical compression. Compared with a high temperature heat pump with mechanical compression, the heat produced by the absorption heat pump is 31% cheaper. From (h, x) and (s, x) diagrams, exergy losses for each component can be determined, and, from these results, an exergetic efficiency of 75% is obtained, which provides the quality index of absorption cycle.     

Evaluation of the performance of an absorption–demixtion heat pump for upgrading thermal waste heat

An absorption/demixtion heat pump is used to overcome the classic heat pump's performance limitation due to the high cost of the amount of energy consumed in the separation process. This heat pump concept uses a partially miscible mixture as working fluid instead a binary miscible mixture. Under low pressure the mixture is a heterogeneous solution of two non-miscible phases in thermodynamic equilibrium but at very different compositions. Then the solution spontaneously splits into two liquid phases without any energy supply. Beyond a critical pressure of dissolution the two earlier non-miscible phases become miscible and then form a homogeneous liquid phase. In these conditions, at high temperature, the more volatile phase is evaporated and its vapor absorbed in the less volatile phase in exactly the same way as in the classical heat pump. The features of this system are presented and a comparison is made against the classic absorption heat pump and the compression heat pump. The different steps of the system are represented by the conventional methods of McCabe and Thiele, and Ponchon–Savarit, to provide a good understanding. We calculated the performance of this system and compared its values against those of the absorption heat pump and the compression heat pump: the COP is respectively 2 and 100 times greater because the separation does not need costly energy. Moreover, the system presents investment cost saving because the separator is a simple decanter.     

Performance characteristics of an ammonia–water absorption heat pump system

The influences of the performance parameters and the heat transfer characteristics of the absorption heat pump using ammonia–water mixture are theoretically carried out. There is a pronounced effect of the ammonia concentration ξ after rectifier on the temperature glides that has been investigated. At ξ = 0.9000 and saturation pressures of 75 and 0.5 bar, the temperature glides are 64.4°C and 81.21°C, respectively, whereas these glides are 0°C and 16.1°C at ξ = 0.9999 and at the same pressures. This mixture property considerably affects the absorption system performance and the design of the rectifier as well as other absorption components. A correlation of the Nusselt number, Nu, is developed and compared with some published work in the literature for plate type heat exchanger. The effects of ammonia concentration ξ, mass fraction spread Δξ, specific solution circulation ratio f, and pressure ratio R_p on the refrigerant mass flow rate, the pressure drop, and the heat transfer coefficients during the condensation, the evaporation, and the absorption processes are investigated. It was found that increasing ammonia mass fraction spread Δξ results in both specific circulation ratio f and R_p that have insignificant effects on the refrigerant mass flow rate. Mounting Δξ at constant f reduces the pressure drop gradually and subsequently starts to increase as Δξ escalates. The ammonia concentration ξ has insignificant effect on the evaporation heat transfer coefficient but has a little effect on the condensation and the absorber heat transfer coefficients. The ammonia mass fraction spread Δξ and f have considerable effects on the heat transfer coefficient for different absorption heat pump components. R_p has a pronounced effect on the evaporation heat transfer coefficient, although it has a slight effect on the condensation and the absorber heat transfer coefficients. The effect of R_p on the heat transfer coefficient may be eliminated in the absorber for Δξ > 0.18. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental study of operating characteristics of compression/absorption high-temperature hybrid heat pump using waste heat

This research describes the development of a compression/absorption hybrid heat pump system that utilizes a mixture of NH₃ and H₂O as a working fluid. The heat pump cycle is based on a hybrid combination of vapor compression cycle and absorption cycle. The system consists of major components of two-stage compressors, absorbers, and a desorber. There are also auxiliary parts like a desuperheater, solution heat exchangers, a solution pump, a rectifier, and a liquid/vapor separator to support stable operation of the heat pump. This compression/absorption hybrid heat pump provides many advantages of performance over conventional vapor compression heat pumps including a large temperature glide, an improved temperature lift, a flexible operating range, and greater capacity control. These benefits are optimized by changing the composition of the mixture. In this study, the effect of the composition on the operating characteristics of the compression/absorption hybrid heat pump was experimentally observed.     

Mass and energy storage analysis of an absorption heat pump with simulated time dependent generator heat input

This communication presents an assessment of the feasibility of energy storage via refrigerant mass storage within an absorption cycle heat pump with simulated time dependent generator heat input. The system consists of storage volumes with the condenser and absorber of the conventional absorption cycle heat pump to store liquid refrigerant, weak and strong solutions during the generation period, which are required for the heat pump operation during the generation off period. A time dependent mass and energy storage analysis based on mass and energy balance equations for various components of the heat pump system has been carried out to evaluate energy storage concentration and storage efficiency for combined and separate storage schemes for the weak and strong solutions. Two possible performance modes, viz constant pumping ratio or the constant flow of the strong solution from the absorber to the generator have been analysed: the latter is preferable over the former from a practical point of view. Numerical computer simulation has been made for a typical winter day in Melbourne (Australia) with the desired heating load specified. It is found that the concept of refrigerant storage within the absorption cycle heat pump is technically feasible for efficient space heating. The energy storage concentration in the condenser store is slighly higher while that in absorber store is slightly lower for the separate storage mode as compared to the combined storage. However, the combined storage has an advantage of less storage volume and hence is more cost effective than separate storage and the disadvantage of limited system operation due to the decrease of solution concentrations.     

Heat pump assisted distillation. V: A feasibility study on absorption heat pump assisted distillation systems

The problems in matching a heat driven absorption heat pump to a distillation process in heat pump assisted distillation are discussed. The performance of an absorption system is a function of the temperatures in the evaporator, the condenser, the absorber and the generator and the ratio of the mass flow rate in the secondary circuit to the mass flow rate in the primary circuit. In absorption systems design choices are limited by the Gibbs phase rule. Plots are given of the coefficient of performance against the temperatures of the top and bottom products and also against the energy saved.     

A high temperature absorption heat pump as topping process for power generation

With a high temperature absorption heat pump as a topping device to the Rankine-water cycle, the efficiency for power generation may be improved by 40–50% above present values. A possible heat-pump process working with water as absorbent and CaO as absorber is discussed by using diagrams which are suited to study the combination of a heat pump with the Rankine cycle.     

Wood chip drying with an absorption heat pump

This paper presents the thermal and economic analysis of a mobile wood chip drying process. The dryer was subjected to different operating conditions, which were studied in order to determine the optimal characteristics of the dryer in terms of energy consumption. In addition, the impact of the exterior climatic conditions on the dryer's performance was also evaluated. The performance of the dryer coupled with an absorption heat pump was modeled in steady-state conditions under different operating parameters. Finally the system's energy performance was compared to the performance of two other systems (a wood burning furnace and a waste-heat recovery system). The results demonstrate that single-stage absorption heat pumps are only interesting when the set point temperature of the drying air is below 60 °C. Otherwise, a two-stage absorption heat pump must be used. In terms of energy and financially, this type of drying is very costly. Of the three processes that were studied, heat recovery systems proved to be the most energy efficient and economic solution.     

Chemical heat pump using heat of reversible catalytic reactions

One of the most important objects of research in the field of energy conservation is development of a high temperature heat pump for the industrial sector. In this framework this paper introduces a new concept of heat pump, which is a hybrid between a compressor and chemical heat pump, where instead of the latent heat of liquids, the proposed system uses the heat of gaseous catalytic reversible reactions (type A → B + C). This paper proposes a simple scheme with a mathematical model for the calculation of the parameters necessary to qualify the behaviour of the Heat Reaction Chemical Heat Pump (HRCHP). The Coefficient of Performance (COP) is calculated as a function of the temperature of the heat source, the temperature to which the pump upgrades the heat and the conversion of the chemical reaction. A more optimized scheme is prospected.     

A mathematical model with experiments of single effect absorption heat pump using LiBr–H2O

A mathematical model of a single effect, LiBr–H₂O absorption heat pump operated at steady conditions is presented. This model took into consideration of crosscurrent flow of fluids for heat and mass exchangers, two-dimensional distribution of temperature and concentration fields, local values of heat and mass transfer coefficients, thermal parameter dependent physical properties of working fluids and operation limits due to the danger of the LiBr aqueous solution hydrates and crystallization. Improvements of the calculation method make this simulation much more convenient and efficient. An improved absorber experiment set-up and a complete absorption heat pump were built and tested for further study. It was found that the mass flux of vapor increased with the increase of absorber pressure, coolant flow rate, spray density of LiBr solution and decrease of coolant and input temperature of solution. And the vapor mass flux increased almost linearly with the increase of absorber pressure. Results derived from this model show agreement within 7% with experimental values.     

Performance assessment of contact heat pump drying

A comparative analysis is presented of a novel plate‐type isothermal heat pump dryer and a conventional adiabatic heat pump dryer. The energy performance and drying capacity of each dryer type is investigated subject to operational constraints on the maximum acceptable relative humidity and temperature of air passing over the product. The analysis demonstrates that for applicable products, a significant opportunity exists for improving the energy efficiency of heat pump drying, by a factor of 2–3 times compared with current adiabatic heat pump dryers. The moisture extraction rate is also increased in the contact heat pump dryer, by a similar factor. However, these improvements are shown to be sensitive both to the impact of product thickness on heat transfer and to the relative humidity constraint. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimum performance of a heat engine‐driven combined vapour compression–absorption–ejector heat pump

Optimum performance of an endoreversible heat engine‐driven heat pump cycle, based on a combination of an absorption cycle with a vapour and ejector compression cycles is investigated. This combination increases the performance of the conventional ejector and absorption cycles and provides high performance for heating. The analysis show that the combined heat pump cycle has a significant increase in system performance over the heat engine‐driven vapour compression or absorption heat pump cycle and heat engine‐driven combined vapour compression and absorption heat pump cycle. Copyright © 2000 John Wiley & Sons, Ltd.     

污水源冷热水机组的热力学分析

阐述了污水源冷热水机组在制冷和热泵工况下的各个设备的火用损失及整个机组的的火用效率计算公式,计算分析了机组在夏季制冷和冬季制热工况下各个设备在不同污水温度下的火用损失系数及整个机组的火用效率。     

The importance of a storage tank in the brine cycle of an absorber roof-assisted heat pump

A dynamical model to calculate the performance of an absorber roof-assisted heat pump is developed. The model is applied to a specific example, a normal residence building in Berlin. The heat pump is layed out to meet the heat demand at 0°C outside temperature. The auxiliary heat is provided by a conventional backup system. Conditions of insolation and outdoor temperature are derived from real weather data and simulated in their dynamical behaviour. The analysis investigates how the variation of the storage tank volume and the lower limiting COP is reflected in the annual cost savings.     

Solar-assisted absorption heat pumps feasibility

An absorption system can be used for space cooling as well as for space heating. This dual purpose may be achieved by using the system as heat pump in wintertime. Absorption heat pump heating may be an interesting alternative, particularly for countries where there is a shortage of electric power.When an absorption unit is used as heat pump, its mode of operation is not modified: the internal temperatures of the cycle are only raised. Commercially available LiBr units were tested as heat pumps. COP and heating capacity were considered as a function of cold source temperature for different temperatures of the useful heat. The COP arrived at 1.7, which must be considered a high value for a thermally driven heat pump.Simulations were carried out in order to compare the performance of “conventional” solar, solar assisted heat pump and the combined series system under two different climate conditions. The series system showed performance 25–75 per cent better than “conventional” solar alone.     

Experimental study on the performance of air source heat pump system with multiple parallel outdoor units

This paper presents a novel air source heat pump for heating of buildings named air source heat pump with multiple parallel outdoor units (ASHPMO). Multiple outdoor units were connected in parallel with the aim of realising alternate defrosting and uninterrupted heating simultaneously. An experimental apparatus of the ASHPMO system was developed. The defrosting performance was experimentally investigated under different outdoor air temperatures, outdoor air relative humidity, and condensation temperatures, among other factors. The test results showed that the novel ASHPMO system could provide continuous heating when defrosting even under an outdoor air temperature of −10°C. Variations in compressor vapour suction and discharge pressure and temperature were observed. The minimum heating capacity could still reach 60% of that without defrosting. Under the defrosting condition with outdoor air temperature −10°C, both the heating coefficient of performance (heating COP) and total energy efficiency ratio (EER) of the system can reach to 2.0 and 2.32, respectively.     

循环水对水环热泵系统运行的影响研究

以循环水为研究对象,采用部分负荷率法建立能量方程并定义无因次能耗指数(Ic、Ih)作为水环热泵系统运行变化的动态指标。结果表明:无因次能耗数与负荷率之间具有线性关系,但应注意:在制热工况下,受热泵性能系数影响,系统运行会产生显著波动并引起能耗升高。选取天津某办公建筑为案例进行测试以检验上述结论,并用循环水动态能量分析方法处理实验数据,结果表明:相比热泵系统,负荷率对机组能耗的影响更显著,特别是在冬季畸变工况段;另外,冬季热泵系统运行会受辅助热源与负荷率双因子影响,这将降低畸变工况段的能耗变化幅度。因此,合理选择辅助热源容量将有助于优化系统整体运行。     

Performance of an experimental ground-coupled heat pump system for heating,cooling and domestic hot-water operation

The ground-coupled heat pump (GCHP) system is a type of renewable energy technology providing space heating and cooling as well as domestic hot water. However, experimental studies on GCHP systems are still insufficient. This paper first presents an energy-operational optimisation device for a GCHP system involving insertion of a buffer tank between the heat pump unit and fan coil units and consumer supply using quantitative adjustment with a variable speed circulating pump. Then, the experimental measurements are used to test the performance of the GCHP system in different operating modes. The main performance parameters (energy efficiency and CO₂ emissions) are obtained for one month of operation using both classical and optimised adjustment of the GCHP system, and a comparative analysis of these performances is performed. In addition, using TRNSYS (Transient Systems Simulation) software, two simulation models of thermal energy consumption in heating, cooling and domestic hot-water operation are developed. Finally, the simulations obtained using TRNSYS are analysed and compared to experimental data, resulting in good agreement and thus the simulation models are validated.     

水环热泵空调系统应用性分析

水环热泵空调系统是小型水／空气热泵机组的一种应用。通过连通建筑物周边区和内区的水循环环路将小型水／空气热泵机组并联在一起，形成一个封闭环路，将建筑物内区制冷产生的冷凝热转移到周边区，作为周边区热泵的低温热源。水环热泵空调系统在对建筑物内区供冷的同时实现对周边区供热，使建筑物空调的内、外区冷热相抵，建筑物内部余热得到充分利用，节约了能源。本文比较了水环热泵系统相对于常规空调系统在系统结构、节省能源等方面的优势和应用中遇到的问题，并结合工程应用实例，分析了水环热泵空调系统的经济性。     

Exergoeconomic performance of an endoreversible Carnot heat pump with complex heat transfer law

The finite-time exergoeconomic performance of an endoreversible Carnot heat pump with a complex heat transfer law, including generalized convective heat transfer law and generalized radiative heat transfer law q∝ (Δ T ⁿ ) ^m , is investigated in this paper. The focus of this paper is to obtain the compromised optimization between economics (profit) and the energy utilization factor (coefficient of performance, COP) for the endoreversible Carnot heat pump, by searching the optimum COP at maximum profit, which is termed as the finite-time exergoeconomic performance bound. The obtained results include those obtained in much of the literature and can provide some theoretical guidance for the design of practical heat pumps.