Modelling and control of an air-to-air heat recovery system

Analytical modelling, controller design and simulation of an air-to-air runaround coil heat recovery system are studied. The system consists of two plate-fin-tube coils, a storage tank, two circulating pumps, an auxiliary heater and associated distribution network. Simulation results showing the transient response characteristics of the open-loop and closed-loop system are given. Results show that the designed controllers are able to hold the preheat temperature and the storage tank temperature at their respective setpoints irrespective of changes in outdoor and exhaust (source heat) temperatures.     

Thermal modelling and energy conservation studies on freon rankine cycle cooling system with regenerative heat exchanger

This communication presents a thermodynamic analysis and assessment of a Freon fluid Rankine cycle cooling system. The system consists of two subsystems—Rankine engine (RE) power cycle and a vapour compression (V-C) refrigeration cycle. The heat engine subsystem consists of a boiler, turbine, condenser and a feed pump while the cooling subsystem consists of a mechanical compressor, condenser, evaporator and an expansion valve. A number of working fluid combinations for the RE cycle and V-C cycle subsystems have been chosen on the basis of their thermodynamic properties and their suitability judged in terms of the performance parameters, namely, the thermal efficiency of the power cycle and the coefficient of performance (COP) of the refrigeration cycle. A regenerative heat exchanger (RHE) is incorporated in the RE cycle to improve the cycle efficiency and achieve energy conservation.The effects of various operation parameters, namely, component temperatures, adiabatic expansion/compression efficiencies and effectiveness of the RHE on the overall COP have been assessed. It is found that R114 + R22 give the best overall system performance and the presence of the RHE improves the system COP significantly. The effect of V-C cycle condenser temperature is more pronounced as compared to that of the RE cycle condenser and similarly the effect of evaporator temperature in the V-C cycle is more pronounced as compared to that of the boiler in the Rankine cycle subsystem.     

Heat pipe heat storage performance

Heat storage offers essential thermal energy saving for heating. A ground heat store equipped with heat pipes connecting it with a heat source and user is considered in the paper. It has been shown that such a heat exchanging system along with a batch energy source meets, to a considerable extent, house heating requirements.     

Digital control of a heat recovery and storage system

The implementation of optimal control trajectories using digital feedback controllers is explored. The system considered is a heat recovery and storage system used in space heating. Since the optimal trajectories considered here are based on forecasted ambient temperatures, a single method of implementing the optimal trajectories using the sensed zone air temperature as feedback signal is presented. Also, an adaptive controller which controls the storage tank temperature is shown to maintain the zone air temperature very close to the set-point.     

On the performance of absorption heat pump-transformers

A simplified mathematical model has been used to compute the performance of an absorption heat pump-transformer (HPT). This two-stage apparatus, which consists of a heat transformer driven by low-temperature cooling water and of a heat pump driven by high-temperature steam, has been compared with a simpler absorption heat pump.Different indices of performance show that the HPT may give a better performance at a competitive cost. In particular, the upgraded heat load can be produced at a markedly higher temperature. The main drawback of the HPT consists of the smaller values of the ratio between upgraded and heat loads.     

Wide band laser heat treatment using pyramid polygon mirror

A wide laser beam optical system for laser heat treatment processing has been designed with a pyramid polygon mirror, which is able to adjust the amplitude and frequency. It consists of a rotating pyramid polygon mirror made of copper and a cooling part that protects the surface of the mirror from damage by heat. The system provides a linear laser heat source with the wide scanning width. This can increase the efficiency of production and quality of products. The CO₂ laser heat treatment has been processed on SNCM220 and SCM440 steels using the developed optical system. The width of specimens between 13 and 17 mm can be hardened for a single pass scanning.     

废热溴化锂吸收式动力、冷、热量梯阶转换系统分析

介绍了一种利用溴化锂溶液的热力学特性,合理安排废热回收的废热溴化锂吸收式动力、冷、热量梯阶转换系统。该系统将其中一部分高品位的能量进行动力转换,其余转换成热量和冷量,实现能量梯阶转换,提高废热能源中可用能的合理利用率。利用自编的计算程序对系统的废热回收量以及能量利用与转换做了详细分析,得出烟气进出口温度对系统的能量转换的影响。     

A control strategy for heat recovery and storage

A simple method of finding approximate control strategies for combined heat recovery and storage processes is presented. The system considered is a space heating system employing a heat pump. The technique is based on energy balance principles and a numerical search method which minimizes the operating cost. Two examples are given to illustrate the procedure.     

Cogeneration versus industrial waste heat

The National Energy Administration has, by direction from the Swedish Government, made a study of the conditions for future small scale cogeneration of electricity and heat in Sweden. Small scale cogeneration is there defined as cogeneration plants not larger than 25 MW_e. The analysis in this paper deals with anenergy system where small scale cogeneration is one of several ways to provide electricity and heat in an industrial area in Avesta, Sweden. The cogeneration heat has to compete with the waste heat from industrial processes and low temperature heat from the river and ambient air. There is also the municipal district heating system to take into consideration for the supply of heat to the area. In this competitive situation one is motivated to make an analysis of the system in order to find out how best to use the different possibilities for energy supply. The time period covered by the analysis is 10 years, from 1988 to 1977.     

Internal energy and specific heat in a ferromagnetic- antiferromagnetic double layers

The internal energy and specific heat of a Heisenberg ferro- antiferromagnetic double-layer system are studied by using spin-wave theory and the retarded Green function method at low temperatures. Numerical results show that the antiferromagnetic intralayer coupling $J_2 $ has an important influence on internal energy and specific heat for a four-sublattice system with antiferromagnetic (or ferrimagnetic) interlayer couplings.     

The second law, Maxwell's demon, and work derivable from quantum heat engines

With a class of quantum heat engines which consists of two-energy-eigenstate systems undergoing, respectively, quantum adiabatic processes and energy exchanges with heat baths at different stages of a cycle, we are able to clarify some important aspects of the second law of thermodynamics. The quantum heat engines also offer a practical way, as an alternative to Szilard's engine, to physically realize Maxwell's demon. While respecting the second law on the average, they are also capable of extracting more work from the heat baths than is otherwise possible in thermal equilibrium.     

Quantum heat engines, the second law and Maxwell's daemon

We introduce a class of quantum heat engines which consists of two-energy-eigenstate systems, the simplest of quantum mechanical systems, undergoing quantum adiabatic processes and energy exchanges with heat baths, respectively, at different stages of a cycle. Armed with this class of heat engines and some interpretation of heat transferred and work performed at the quantum level, we are able to clarify some important aspects of the second law of thermodynamics. In particular, it is not sufficient to have the heat source hotter than the sink, but there must be a minimum temperature difference between the hotter source and the cooler sink before any work can be extracted through the engines. The size of this minimum temperature difference is dictated by that of the energy gaps of the quantum engines involved. Our new quantum heat engines also offer a practical way, as an alternative to Szilard's engine, to physically realise Maxwell's daemon. Inspired and motivated by the Rabi oscillations, we further introduce some modifications to the quantum heat engines with single-mode cavities in order to, while respecting the second law, extract more work from the heat baths than is otherwise possible in thermal equilibria. Some of the results above are also generalisable to quantum heat engines of an infinite number of energy levels including 1-D simple harmonic oscillators and 1-D infinite square wells, or even special cases of continuous spectra.     

Waste heat recovery using looped heat pipes for air cooling

A scheme is described for the recovery of waste heat from stacks of gas turbine engines and the utilization of recovered energy for the cooling of ambient air. Relationships are summarized for the modeling of components of the cooling system. Samples are presented from performance data that is predicted by the model. Effect of size and design of system components, as well as operational variables on system performance, are discussed. It is concluded that the single most significant variable in the design of the looped heat-pipe recovery and utilization system is the geometry of the exhaust pipe of the gas turbine engine. Accordingly it is suggested that a design for the exhaust pipe of a gas turbine must consider the effects of (a) the variation of velocity of exhaust gases at different exhaust inlet temperatures, and the consequent pressure drops in the exhaust chimney pipe, and (b) the length of the exhaust pipe. The latter essentially determines the length of the heat pipe evaporator. Furthermore, the temperature drop through the air cooler is also significant, since this also influences system performance.     

Economic comparison between adsorption and compression heat pumps

Adsorption and electric heat pumps have been compared for domestic heating and cooling for three Italian cities: Messina, Rome and Milan. The daily energy needs based on the characteristics of a flat and on the meteorological data of the cities have been calculated. From heating and cooling efficiencies of the electric and adsorption heat pumps the electric and gas energy consumption have been evaluated and compared. Finally, the annual cost of the electric heat pump has been estimated and from this, the maximum available cost of the adsorption machine is presented for each city.The final results show that adsorption heat pumps represent an energy saving machine with a good economic competitivity with respect to the traditional system.     

Failure of free energy relation under non-Markovian heat bath temperature change

We investigate the free energy relation for a system contacting with a non-Markovian heat bath and find that the validity of the relation sensitively depends on the non-Markovian memory effect, which is especially related to the initial preparation effect. This memory effect drives the statistical distribution of the system out of the initial preparation, even if the system starts from an equilibrium state. This leads to the violation of the free energy relation. A possible way of eliminating this memory effect is proposed.     

Study on the concentration difference heat pump using fusion and freezing processes of acetic acid-acetamide system

The concentration difference heat pump using fusion and freezing processes to generate a cold fluid has been investigated. This heat pump system utilizes the acetic acid-acetamide pair as working material, and consists of a cold fluid generating and a separation process. The operation at the cooling capacity of 3.52 kW (1 ton of refrigeration) has been investigated in this study. At the cold fluid generating process, solid acetic acid at 15°C is fused into an acetamide solution at 15°C, such that the temperature and the concentration of acetamide of the solution decreases. This dilute solution at lower temperature can be used to generate a cold fluid. The lowest attainable temperature of the solution has been investigated experimentally, and also calculated from the energy balance equation. The decreasing rates of the temperature have also been studied. At the separation process, continuous distillation is adopted to concentrate the dilute solution sent from the cold fluid generating process. The data which support the possibility of separation by continuous distillation are presented. The energy demand at the separation process is investigated theoretically.     

Economic application of heat pumps in integrated distillation systems

The influence of relevant parameters on the economics of distillation plants involving distinct heat pump cycles is scrutinised and the results are compared to conventional and integrated schemes. On the basis of the COP, energy costs and efficiencies, simple expressions are proposed for preliminary economic analysis and design of heat pump assisted distillation. The influence of heat pump type, purity requirement, column pressure drop, feed rate, energy cost, relative volatility etc. on the energetic aspects and economic range of application is presented. Heat pumps can be economical substitutes for conventional distillation process design whenever direct refrigeration or chilled water are required for condensation and for separating close boiling mixtures in columns of small pressure drop. A design strategy for selecting the most economical distillation system, considering different types of heat pump structures (vapour recompression, bottom flash, closed cycle, absorption cycle), is proposed, based on pinch technology, primary energy rate, energy cost factor and estimated payback time of excess capital. The strategy is demonstrated by industrial case studies.     

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.