Solar space heating and cooling with bi-heat source heat pump and hot water supply system

This paper describes a specific heat pump system that can solve the problem of low heating capacity at a low ambient temperature—one of the largest problems in the air-source heat pump system.

In order to decrease the collector area required, the heat pump system is operated by the air-source during the daytime, but at night or at a very low ambient temperature it can be operated with hot water which has been produced by the collector in the daytime. The effect of the solar energy on the air-source heat pump system has many advantages in the moderate winter climate of Japan. The hot water supply system includes an auxiliary electric heater.

The experiment has been carried out with a prefabricated test house, which has been constructed in Nara with double glazed windows and high thermal insulation. The results of this experiment are that solar energy enhances the total electric energy savings, increases the heating capacity at low ambient temperature, and eliminates the need for reverse cycle defrosting operation, etc.     

Ionic liquid based absorption chillers for usage of low grade waste heat in industry

The standard working pairs for absorption chillers, ammonia/water and water/lithium bromide show problematic behaviours like crystallisation and corrosiveness. Because of their convenient solving properties and their low vapour pressure, ionic liquids are a new promising class of sorbents for absorption cooling purposes. In this study, the working pairs water/1,3‐dimethylimidazolium dimethylphosphate ([MMIM][DMP]) and water/1‐ethyl‐3‐methylimidazolium dimethylphosphate ([EMIM][DMP]) are implemented in AspenPlus. The performance of a single effect cycle with these pairs is simulated and compared to results of a cycle with water/LiBr. For [EMIM][DMP] a coefficient of performance (COP) comparable to that of LiBr or even higher (up to 0.85) is found. [MMIM][DMP] shows a smaller maximum COP but a largely wider operating temperature range than LiBr. Results are compared with those of other groups, discrepancies discussed and improvements suggested. Copyright © 2013 John Wiley & Sons, Ltd.     

Analysis and assessment of a nanoparticle seeded small scale absorption refrigeration system driven by a low‐grade waste heat source

The thermoeconomic behaviour of a nanoparticle seeded single effect LiBr‐H₂O absorption refrigeration system (ARS) is investigated for a small scale application. In the proposed method, alumina nanoparticles with volume concentrations of 3%, 5%, and 7% are dispersed into an aqua lithium bromide solution. The multiobjective heat transfer search algorithm is employed to examine the design trade‐off between the coefficient of performance (COP) and total annualized cost (TAC). To analyze the overall performance of the system, the influence of five design parameters, namely the temperatures of the generator, absorber, evaporator, condenser and heat exchanger pipe diameter, are studied. It is found that with an increase in the COP, the TAC of the system is initially raised marginally, and after that, raised rigorously with further increment. The comparative results indicate that the COP and TAC of the nanofluid based ARS system are increased by about 7% and decreased by about 3.2%, respectively, corresponding to the Pareto points of the base ARS system. A lower break‐even point of about 2.6 years is achieved for the ARS system containing nanoparticles compared to the base ARS system. Overall, the ARS system containing 5% nanoparticles is the best solution from a thermodynamic and economic point of view.     

Solar energy development in China—A review

The steady and maintainable electric power provides the development momentum of a country's industrialization, which is indispensable to every country at present. It is well known that China is the largest developing country in the world. With the rapid development of economy and society, energy demand of Chinese society is increasing in an incredible speed, i.e., the annual accumulative total capacity of electric energy is about 0.1 billion kW, and the most of them is provided by the fossil fuel resource, and the share is about 90% in China. Certainly, it is a very inappropriate energy structure, so the sustainable development of country is impossible in future, the status must be improved in order to achieve sustainable development. Fortunately, China has large country area, and there are abundant solar resources. Development and application of solar energy have been regarded by the government and ordinary people, and they thought that solar energy can provide more and more electric energy in future, and more and more actual examples have been applied in the last decades, which are supported by central government and local governments. This paper discusses the distribution zone and current developmental situation of solar energy in China. Then, some application practice is described, such as solar energy greenhouse, solar energy hearth, solar water heater, solar lighting system, solar water pump, distributed generation (DG), grid-connect photovoltaic generation (GPG) and wind–solar hybrid system. The policies and law of China central government and local governments are described in the following paragraph. At the end, the developmental prospect of photovoltaic (PV) in future China and the development barriers and recommendations are introduced.     

Analysis of a combined power and cooling cycle for low‐grade heat sources

This paper presents a parametric analysis of a combined power/cooling cycle, which combines the Rankine and absorption refrigeration cycles, uses ammonia–water mixture as the working fluid and produces power and refrigeration, while power is the primary goal. This cycle, also known as the Goswami Cycle, can be used as a bottoming cycle using waste heat from a conventional power cycle or as an independent cycle using low‐temperature sources such as geothermal and solar energy. Optimum operating conditions were found for a range of ammonia concentration in the basic solution, isentropic turbine efficiency and boiler pressure. It is shown that the cycle can be optimized for net work, cooling output, effective first law and exergy efficiencies. The effect of rectification cooling source (external and internal) on the cycle output was investigated, and it was found that an internal rectification cooling source always produces higher efficiencies. When ammonia vapor is superheated after the rectification process, cycle efficiencies increase but cooling output decreases. Copyright © 2010 John Wiley & Sons, Ltd.     

District cooling and heating with seawater as heat source and sink in Dalian, China

This paper presents a study on the utilization of ocean thermal energy in Dalian. Coastal areas are ideal sites for the application of seawater-source heat pump technology (SWHP) to provide district cooling and heating. The technical requirements and the scheme design in the context of marine climate are discussed. A government-commissioned feasibility study of a proposed district energy site in Dalian, with an estimated 68 MW heating load and 76 MW cooling load plant capacity is then described. The economic, energy and environmental impacts are analyzed. In order to get a better understanding of economic justification and benefits of two different systems, annual cost (AC) and net present value (NPV), are used. A series of sensitivity analyses are undertaken to investigate the magnitude of the effect of the parameters variation on cost calculation. The numerical simulations of seawater temperature field are performed by using a two-dimensional convection–diffusion equation model to evaluate environmental impacts. Study indicates that Dalian has great potential for applying SWHP system. It is both technically and economically feasible because of the favorable geographical location and urban environment.     

Wind energy development in China — Reality and market forces

Economic reforms in China started in 1978, which led to profound changes as a result of a consistent structural adjustment and stabilisation policy. The national economy is now characterised with high growth and low inflation. In 1997, GDP was US$ 767 billion and foreign exchange reached US$ 140 billion. This paper examines the outstanding contribution of rural industries to rapid growth of national economy and the consequences of increase of energy consumption and its environmental impact. It also emphasises the necessity and benefit of using renewable energy and wind energy in particular. The paper also addresses the issue of joint venture in farm development in line with Chinese market economy.     

Studies on a compression‐absorption heat pump for simultaneous cooling and heating

A thermodynamic analysis has been carried out on a compression‐absorption system that can be used for simultaneous heating and cooling applications. Refrigerant and absorbent pair of R‐22 and DMETEG has been considered. The results are obtained for a system that can be used in dairy plants for pasteurization and chilling of milk for subsequent storage. Results show that by operating the system at marginally higher discharge pressures (about 30 bar) than normal (about 25 bar) one can achieve excellent performance despite the required large temperature lift. A comparison with a single stage vapour compression system shows that, the compression‐absorption system yields much better overall performance especially when the temperature lifts are high. Copyright © 2004 John Wiley & Sons, Ltd.     

吸收式制冷循环利用低品位热能的研究现状和发展趋势

介绍吸收式制冷循环利用低品位热能的研究现状和发展趋势。阐述吸收式制冷系统对太阳能、工业余热、生物质能和地热能四种能源的利用情况,并主要从低品位热能的选择、吸收式制冷循环系统的优化和吸收器的优化三方面分析该系统存在的问题和发展趋势。     

Thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement for a low‐grade heat source

A thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement using a low‐grade industrial waste heat is presented. Under the identical operating conditions, the reheat cycle is compared to the non‐reheat baseline cycle with respect to the specific net power output, the thermal efficiency, the heat exchanger area, and the total capital costs of the systems. Detailed parametric effects are investigated in order to maximize the cycle performance and minimize the system unit cost per net work output. The main results show that the value of the optimum reheat pressure maximizing the specific net work output is approximately equal to the one that causes the same expansion ratio across each stage turbine. Relative performance improvement by reheat process over the baseline is augmented with an increase of the high pressure but a decrease of the turbine inlet temperature. Enhancement for the specific net work output is more significant than that for the thermal efficiency under each condition, because total heat input is increased in the reheat cycle for the reheat process. The economic analysis reveals that the respective optimal high pressures minimizing the unit heat exchanger area and system cost are much lower than that maximizing the energy performance. The comparative analysis identifies the range of operating conditions when the proposed reheat cycle is more cost effective than the baseline. Copyright © 2012 John Wiley & Sons, Ltd.     

Organic Brayton Cycles with solid sorption thermal compression for low grade heat utilization

While organic Rankine cycles have been widely used for power generation using low grade thermal energy, Brayton cycles have not been considered feasible because the work required to compress the gas nearly compensates the turbine work output. However, if the low grade energy can be used for thermal compression of the working fluid, it may be possible to gainfully operate the Brayton cycle. With this in mind, a solid sorption based Brayton cycle is proposed in this paper. R134a, CO₂, R507a, propane, R32 and R410a with activated carbon as sorbent, were considered in this proof-of-concept study due to the ready availability of adsorption data. Even though the thermal efficiency is low (<8%), the proposed scheme could add an option for distributed power generation using solar or waste heat. It is found that if irreversibilities in turbine and thermal compression are considered R32 gives a better performance than CO₂ and R410a.     

Shallow gravel aquifers and the urban ‘heat island’ effect: a source of low enthalpy geothermal energy

Northern European countries with no high temperature geothermal resources can utilise the urban ‘heat island’ effect to generate low enthalpy geothermal energy for space heating/cooling systems in buildings, provided a suitable aquifer underlies the urban area. Buried valleys, formed at the height of the Pleistocene glaciation 15,000 years ago, when sea level was 130 m lower than present, and infilled with gravels as sea level rose again at the end of the Pleistocene, underlie many European cities. These high yielding aquifers exist at only a few metres depth, and can provide a supply of groundwater at temperatures elevated 3–4 K above the average rural groundwater temperatures. This can produce a marked improvement both in the output and in the efficiency of a geothermal system making use of this source. When passed through a heat pump operating at a Coefficient of Performance (COP) of 4.5:1, a well yielding 20 l/s of groundwater at 13 °C can generate 865 kW heat, sufficient to supply space heating for buildings with a footprint in excess of 12,000 m² with a peak heating intensity of 70 W/m². The economics of this low enthalpy geothermal energy source are outlined. Although development costs are minimal, at current low natural gas fuel prices in Ireland, heating-only applications will be less attractive, and a real cost saving will only accrue if dual heating/cooling functions can be developed.     

On the clear sky model of the ESRA — European Solar Radiation Atlas — with respect to the heliosat method

This paper presents a clear-sky model, which has been developed in the framework of the new digital European Solar Radiation Atlas (ESRA). This ESRA model is described and analysed with the main objective of being used to estimate solar radiation at ground level from satellite images with the Heliosat method. Therefore it is compared to clear-sky models that have already been used in the Heliosat method. The diffuse clear-sky irradiation estimated by this ESRA model and by other models has been also checked against ground measurements, for different ranges of the Linke turbidity factor and solar elevation. The results show that the ESRA model is the best one with respect to robustness and accuracy. The r.m.s. error in the estimation of the hourly diffuse irradiation ranges from 11 Wh m⁻² to 35 Wh m⁻² for diffuse irradiation up to 250 Wh m⁻². The good results obtained with such a model are due to the fact that it takes into account the Linke turbidity factor and the elevation of the site, two factors that influence the incoming solar radiation. In return, it implies the knowledge of these factors at each pixel of the satellite image for the application of the Heliosat method.     

城市污水作为低品位热源的分析及其热泵辅助系统的针对性配置

城市污水作为热泵供热系统的低品位热源已经为人们所普遍接受。然而,污水的非牛顿流问题以及污水动力粘度高最终会造成其输送成本偏高、传热系数降低等问题,这需要在系统设计时对其做针对性思考。实践证明,与较高可靠性相适应的污水热泵机组的辅助系统应包括：污水沉降系统、污水阻污器、污垢清洗装置和前置换热器等。     

Thermodynamic and thermoeconomic optimization of a cooling tower-assisted ground source heat pump

Thermodynamic and thermoeconomic optimization of a cooling tower-assisted ground source heat pump (GSHP) in a multi-objective optimization process is performed. A thermodynamic model based on energy and exergy analyses is presented, and an economic model of the hybrid GSHP (HGSHP) system is developed according to the total revenue requirement (TRR) method. The proposed hybrid cooling tower-assisted GSHP system, including 12 decision variables, is considered for optimization. Three optimization scenarios, including thermodynamic single objective, thermoeconomic single objective, and multi-objective optimizations, are performed. In multi-objective optimization, both thermodynamic and thermoeconomic objectives are simultaneously considered. An optimization process is performed using the genetic algorithm (GA). In the case of multi-objective optimization, an example of a decision-making process for selection of the final solution from the Pareto optimal frontier is presented. The results obtained using the various optimization approaches are compared and discussed. Further, the sensitivity of optimized systems to the interest rate, the annual number of operating hours in cooling mode, the electricity price, and the water price are studied in detail. It is shown that the thermodynamic optimization is focused on provision for the limited source of energy, whereas the thermoeconomic optimization only focuses on monetary resources. In contrast, the multi-objective optimization considers both energy and monetary. Further, it is found that thermodynamic optimization is economical when the operating time in cooling mode is long and/or the electricity price is high, and water prices variations have no marked impact on the total product cost.     

European perspective on absorption cooling in a combined heat and power system – A case study of energy utility and industries in Sweden

Mankind is facing an escalating threat of global warming and there is increasing evidence that this is due to human activity and increased emissions of carbon dioxide. Converting from vapour compression chillers to absorption chillers in a combined heat and power (CHP) system is a measure towards sustainability as electricity consumption is replaced with electricity generation. This electricity produced in Swedish CHP-system will substitute marginally produced electricity and as result lower global emissions of carbon dioxide. The use of absorption chillers is limited in Sweden but the conditions are in fact most favourable. Rising demand of cooling and increasing electricity prices in combination with a surplus of heat during the summer in CHP system makes heat driven cooling extremely interesting in Sweden. In this paper we analyse the most cost-effective technology for cooling by comparing vapour compression chillers with heat driven absorption cooling for a local energy utility with a district cooling network and for industries in a Swedish municipality with CHP. Whilst this case is necessarily local in scope, the results have global relevance showing that when considering higher European electricity prices, and when natural gas is introduced, absorption cooling is the most cost-effective solution for both industries and for the energy supplier. This will result in a resource effective energy system with a possibility to reduce global emissions of CO₂ with 80%, a 300% lower system cost, and a 170% reduction of the cost of producing cooling due to revenues from electricity production. The results also show that, with these prerequisites, a decrease in COP of the absorption chillers will not have a negative impact on the cost-effectiveness of the system, due to increased electricity production.     

Numerical study of natural convection cooling of horizontal heat source mounted in a square cavity filled with nanofluid

This paper presents a numerical study of natural convection cooling of a heat source horizontally attached to the left vertical wall of a cavity filled with copper-water nanofluid. The left vertical wall is kept at the constant temperature, while the other ones are kept adiabatic. The numerical approach is based on the finite volume method with a collocated grid arrangement. The SIMPLE algorithm is used for handling the pressure velocity coupling. In this study, the influence of some effective parameters such as: Rayleigh number, location and geometry of heat source and solid concentration are studied and discussed. Results are presented in the form of streamlines, isotherms, and average Nusselt number. The results show that dimension of the heat source is an important parameter affecting the flow pattern and temperature field, so that the average Nusselt number decreases with an increase in the length of the heater. It is also observed that at a given Rayleigh number and definite heat source geometry, the average Nusselt number increases linearly with the increase in the solid volume fraction of nanofluid. The increase of Rayleigh numbers strengthens the natural convection flows which leads to the decrease in heat source temperature. The algorithm and the computer code have been also compared with numerical results in order to verify and validate the model.     

Experimental study of convective heat transfer during cooling with low air velocity in a stack of objects

During cooling with low air velocity (u0.2 m·s⁻¹) of a stack of foodstuffs (a few centimeters dimension), the radiation and conduction between products can be of the same order of magnitude as convection. A method was developed to quantify these various transfer modes. The experiment was carried out using an in-line spherical arrangement; however, the same methodology can be applied to other product shapes. The results confirm that the heat transfers by radiation and conduction cannot be neglected. In addition, the convective heat transfer coefficient varies not only with air velocity but also with the product position in the stack.     

Solar reforming of methane in a direct absorption catalytic reactor on a parabolic dish: I—Test and analysis

The concept of solar driven chemical reactions in a commercial-scale volumetric receiver/reactor on a parabolic concentrator was successfully demonstrated in the CAtalytically Enhanced Solar Absorption Receiver (CAESAR) test. Solar reforming of methane (CH₄) with carbon dioxide (CO₂) was achieved in a 64 cm diameter direct absorption reactor on a parabolic dish capable of 150 kW solar power. The reactor was a catalytic volumetric absorber consisting of a multilayered, porous alumina foam disk coated with rhodium (Rh) catalyst. The system was operated during both steady-state and solar transient (cloud passage) conditions. The total solar power absorbed reached values up to 97 kW and the maximum methane conversion was 70%. Receiver thermal efficiencies ranged up to 85% and chemical efficiencies peaked at 54%. The absorber performed satisfactorily in promoting the reforming reaction during the tests without carbon formation. However, problems of cracking and degradation of the porous matrix, nonuniform dispersion of the Rh through the absorber, and catalyst deactivation due to sintering and possible encapsulation, must be resolved to achieve long-term operation and eventual commercialization.     

Solar reforming of methane in a direct absorption catalytic reactor on a parabolic dish: II—Modeling and analysis

The CAtalytically Enhanced Solar Absorption Receiver (CAESAR) test was conducted to determine the thermal, chemical, and mechanical performance of a commercial-scale, dish-mounted, direct catalytic absorption receiver (DCAR) reactor over a range of steady state and transient (cloud) operating conditions. The focus of the test was to demonstrate “proof-of-concept” and determine global performance such as reactor efficiencies and overall methane conversion. A numerical model was previously developed to provide guidance in the design of the absorber. The one-dimensional, planar, and steady-state model incorporates the following energy transfer mechanisms: solar and infrared radiation, heterogeneous chemical reaction, conduction in the solid phase, and convection between the fluid and solid phases. Improvements to the model and improved property values are presented here. In particular, the solar radiative transfer model is improved by using a three-flux technique to more accurately represent the typically conical incident flux. A spatially varying catalyst loading is incorporated, convective and radiative properties for each layer in the multilayer absorber are determined, and more realistic boundary conditions are applied. Considering that this test was not intended to provide data for code validation, model predictions are shown to generally bound the test axial thermocouple data when test uncertainties are included. Global predictions are made using a technique in which the incident solar flux distribution is subdivided into flux contour bands. Reactor predictions for anticipated operating conditions suggest that a further decrease in optical density (i.e., extinction coefficient) at the front of the absorber inner disk may improve absorber conditions. Code-validation experiments are needed to improve the confidence in the simulation of large-scale reactor operation.