Chiller Research Articles

Modelling and dynamic simulation of a hybrid liquid desiccant system regenerated with solar energy

The combination of liquid desiccant systems with conventional vapour compression chillers, usually known as hybrid liquid desiccant systems (HLDS), is a promising alternative when temperature and humidity need to be controlled in air conditioning applications. One of the advantages of this technology is that different kinds of energy can be integrated, particularly low temperature solar thermal energy, which can reduce the electrical consumption of the system. These kinds of systems are typically analysed by discrete steady-state simulations, which show how the system behaves in design conditions. However, dynamic simulations can provide information about the seasonal performance and help to set an appropriate control strategy. This paper describes the modelling and dynamic simulation of an HLDS using Trnsys. Because there are non-standard components for the main elements of a liquid desiccant subsystem (LDS), an alternative modelling method based on performance tables has been developed. The simulation is carried out for Kuala Lumpur, a city with high humidity and ambient temperatures, where air conditioning is required throughout the year. The control strategy is also defined. Finally a sensitivity analysis is performed for the case analysed.

Optimal design and operation of district heating and cooling networks with CCHP systems in a residential complex

The aim of the paper is to describe a mixed integer linear programming (MILP) model for determining the optimal capacity and operation of seven combined cooling, heating and power (CCHP) systems in the heating and cooling network of a residential district (Shahid Beheshti Town) located in east of Tehran. In this model, by minimizing the initial and operating costs of energy supply system, capacity of various components, and optimal operation strategy are determined using continuous variables, and binary decision variables describe the existence/absence of each considered component and its on/off operation status. A distributed energy supply system is made up of district heating and cooling networks, gas turbines as the prime mover for CHP systems, photovoltaic systems (PV) and conventional equipments also, such as boilers and absorption and compression chillers. In this configuration, the required heat is obtained from natural gas consumption by cogeneration system and auxiliary boilers. Four different scenarios were defined to assess the impact of the use of photovoltaic and CCHP systems in the energy supply system in the residential complex. The economic and environmental results obtained from the scenarios revealed saving in costs and reduction in CO2 emissions in the optimal cogeneration system compared with using boilers to produce heat and of buying electricity from the grid.

Modeling and optimizing of a combined CHP system, compression chiller and reverse osmosis plant (CHP + C + W) in two strategies of connections with grid

Energy and environment crises have led contemporary modern human to think about designing systems with high efficiency to meet the energy needs. A Combined Cooling, Heating and Power system with precise utilization of energy dissipation and considering the annual loads reduces fuel consumption and environmental pollution. On the other hand, fresh water is another need of contemporary societies which consume significant amounts of energy especially in hot regions. In this paper, the performance of a CHP system coupled to a Compression chiller, and a plant for fresh Water production, Reverse Osmosis desalination, for domestic uses is investigated. The performance of this system (CHP + C + W) is studied by triple-aspect function (energy, economy and environment) and it was conducted under two conditions of selling (sell, extra electricity generated can be sold to the grid,) and not selling (not sell, extra electricity generated cannot be sold to the grid) electricity for selecting one or two prime movers. The results showed that 70 and 45 kW capacities are required while selecting one prime mover and two prime movers in “sell” mode and “not sell” mode, respectively, to maximize objective function, leading to noticeably reduced system costs. Results showed that maximum annual profit happens in 45 kW (in one prime mover selection) capacity and 2 × 40 (in two prime mover selection) in “not sell” mode and 70 kW capacity and 2 × 30 kW in “sell” mode, respectively; when two prime movers are selected, maximum annual profit declines. Generally speaking, one gas engine selection can be considered as the most appropriate economic option for “sell” and “not sell” modes; however, system performance in two prime mover selection is better concerning load curve coverage and utilization.

Scheduling of Multiple Chillers in Trigeneration Plants

The scheduling of both absorption cycle and vapour compression cycle chillers in trigeneration plants is investigated in this work. Many trigeneration plants use absorption cycle chillers only but there are potential performance advantages to be gained by using a combination of absorption and compression chillers especially in situations where the building electrical demand to be met by the combined heat and power (CHP) plant is variable. Simulation models of both types of chillers are developed together with a simple model of a variable-capacity CHP engine developed by curve-fitting to supplier’s data. The models are linked to form an optimisation problem in which the contribution of both chiller types is determined at a maximum value of operating cost (or carbon emission) saving. Results show that an optimum operating condition arises at moderately high air conditioning demands and moderately low power demand when the air conditioning demand is shared between both chillers, all recovered heat is utilised, and the contribution arising from the compression chiller results in an increase in CHP power generation and, hence, engine efficiency.

<b>Comparative study of air conditioning systems with vapor compression chillers using the concept of green buildings

This paper sets out to compare two different cooling systems that use vapor compression chillers for air conditioning environments. It was proposed to compare different operations in isolated and combined action operations. These operations are evaluated in the concepts of green buildings. A mathematical model was developed based on the principles of mass and energy conservation and complemented by various functions so as to determine the thermophysical properties and efficiencies of the compressors. The equations of the model were solved by the EES (Engineering Equation Solver) program. The model evaluates the influence of the main HVAC operating parameters of the chilled water system when operating under three different configurations. The results showed that the system with a differentiated compression presents a COP equal to that of the system with screw chillers in the range 0-300 RTs, and a COP hat is on average 9% higher in the range 400-800 RTs.

Energy, exergy, economic and environmental (4E) analyses based comparative performance study and optimization of vapor compression-absorption integrated refrigeration system

Present work compares the performance of commercially available 170 kW vapor compression chiller with equivalent three configurations (parallel, series and combined series-parallel) of VCAIRS (vapor compression-absorption integrated refrigeration system) based on combined energy, exergy, economic and environmental (4E) analyses. Parallel, series and combined series-parallel configurations reduces the energy (electricity) consumption in the compressor by 50%, 76.8% and 88.3% respectively and consequently, reduce the significant amount of CO2 emission. Comparative exegetic analysis based on modified Gouy–Stodola law was performed which predicted higher irreversibility rate as compared to conventional approach. The thermoeconomic study shows that annual cost of the plant operation is 13.8%, 20.9% and 24.7% less for parallel, series and combined series-parallel configurations respectively as compared to equivalent VCRS (vapor compression refrigeration system) and after optimization, the same is further reduced by 8.1%, 8.5% and 4.7% respectively from the base value.

Transient simulation and parametric study of solar-assisted heating and cooling absorption systems: An energetic, economic and environmental (3E) assessment

This paper presents energetic, economic, and environmental (3E) analyses of four configurations of solar heating and cooling (SHC) systems based on coupling evacuated tube collectors with a single-effect LiBr–H2O absorption chiller. In the first configuration (SHC1), a gas-fired heater is used as the back-up system, while a mechanical compression chiller is employed as the auxiliary cooling system in the second configuration (SHC2). The capacity of the absorption chiller is designed based on the maximum building cooling load in these configurations. The third and fourth configurations (SHC3 and SHC4) are similar to SHC2, but the absorption chiller size is reduced to 50% and 20%, respectively. The results show that the highest primary energy saving is achieved by SHC2, leading to a solar fraction of 71.8% and saving 54.51% primary energy as compared to a reference conventional HVAC system. The economic performance of all configurations is still unsatisfactory (without subsidies) due to their high capital costs. However, if a government subsidy of 50% is considered, the results suggest that SHC4 can be economically feasible, achieving a payback period of 4.1 years, net present value of 568,700 AUD and solar fraction of 43%, contributing to 27.16% decrease in the plant primary energy consumption.

Theoretical Modelling of Silica Gel Desiccant Wheels

Desiccant evaporative cooling system is a potential environment-friendly alternative to conventional vapour compression chillers. A desiccant wheel is the heart of this heat driven cooling system and it uses a solid desiccant for dehumidification with silica gel being most widely used. The desiccant material is coated, impregnated or formed in a place on the supporting rotor structure. The matrix consists of multiple channels in the direction of axis of rotation of the wheel. Proper design of the wheel is very important for the successful operation of the system and theoretical models are very useful tool in predicting the performance and design optimization. One of the major difficulties in numerical modelling of the desiccant wheel is the lack of knowledge of adsorption equilibrium of porous medium.In this paper, adsorption equilibria of the silica gel is studied and the different theories existing to characterize the adsorption process and different equations for prediction of adsorption equilibria are compared and used in a theoretical model predicting the wheel performance. The results of the different models are compared with experimental data and conclusions are drawn.

Off grid diesel engine-based trigeneration systems with thermal storage and compression chiller for enhanced performance

The paper proposes a diesel engine-based trigeneration system using a thermal storage for applications with simultaneous heating, cooling and power demand and where grid electricity is not available. A thermodynamic analysis is presented for the proposed trigeneration systems considering an 815 kWe diesel generator (DG) as a prime mover. Two configurations are considered. One configuration includes absorption chiller with an auxiliary hot water heater, while the other has a compression chiller in addition to the absorption chiller and water heater. Methodology of integrating a compression chiller is also presented. An analysis of both these options on primary energy savings basis for the considered DG shows that integrating a thermal storage and compression chiller enhances trigeneration performance. Without the compression chiller, the trigeneration system efficiency improves from 46% to 52% while with compression chiller integration, the improvement is much more from 46% to 72% when the power load decreases from 100% to 25%.

Comparison of photovoltaic and solar thermal cooling systems for office buildings in different climates

Photovoltaic systems combined with electrical compression chillers offer a high potential for energy efficient cooling with a high economic feasibility. Due to much higher energy efficiency ratios of electric chillers compared to sorption cooling systems, the heat rejection system is much smaller and thus auxiliary energy and water consumption lower than in sorption cooling systems. The overall system efficiency, auxiliary energy consumption and achievable solar fraction depend on the photovoltaic module technology, the compression chiller energy efficiency ratio and obviously on the temporal correspondence of solar cooling production and cooling demand. A systematic simulation study was carried out to evaluate the overall performance of photovoltaic compression cooling systems in office buildings for different climatic conditions worldwide. For each location and cooling demand the solar energy production for different surface areas was calculated and solar fractions, EER and auxiliary energy demand was determined. It could be shown that the primary energy savings for solar electric cooling and heating are comparable to solar thermal systems. As solar thermal systems include hot storage, they mostly provide higher solar fractions and in some cases higher primary energy savings. If electricity export to the grid or for appliance use is included in the primary energy analysis, photovoltaic cooling systems always have higher primary energy savings.The total cooling costs for solar electric cooling are comparable to solar thermal cooling systems, if there is no feed in tariff for the excess PV electricity. If a grid export is possible and paid for, solar electric cooling systems are always more advantageous.

Optimal planning of energy hubs in interconnected energy systems: a case study for natural gas and electricity

In an energy hub, each energy carrier can be converted to other forms of energy to meet electricity, heating and cooling power demand in an optimal manner. In this study, a framework is presented to optimally design and size interconnected energy hubs. It considers physical constraints on natural gas and electricity networks and environmental issues. The proposed design methodology decides on which components should be allocated to each hub and in what capacity. It includes combined heat and power, boiler, absorption chiller, compression chiller, electricity storage (Li-ion battery) and heat storage. The model also considers incentive policies to install distributed generation thus reducing emissions. Furthermore, it takes energy supply reliability based on availability of components into account. This model can help with conducting studies related to planning future energy systems with interconnected energy hubs. The proposed model has been simulated on an interconnected test system, which represents a municipal district with three energy hubs.

Experimental Investigation of A Liquid Desiccant System for Air Dehumidification Working With Ionic Liquids

Abstract Electrically-driven compression chillers are the commonly used technology for cooling and dehumidifying air. Open sorption systems driven by solar heat are an alternative to conventional air dehumidification technology and may reduce primary energy consumption. For air dehumidification, liquid desiccant systems may exhibit some process engineering and thermodynamic advantages in comparison to solid desiccant systems. The liquid desiccant must exhibit low equilibrium water vapour pressures at the available heat rejection temperature level to achieve low air dew point temperatures and thus a strong air dehumidification with comparably low driving temperatures. Desiccant mass fractions should be as low as possible, but in order to achieve low vapour pressures required desiccant mass fractions may surpass the solubility limit. In the paper at hand, first experimental results of an internally cooled and heated, open liquid desiccant system working with an ionic liquid designed for air dehumidification are presented. It is demonstrated that ionic liquids designed according to the boundary conditions of the respective application may be a promising alternative to commonly used desiccants such as lithium chloride for solar air dehumidification with comparably high heat rejection and low driving temperatures.

Development and analysis of sustainable energy systems for building HVAC applications

The main HVAC applications considered in this paper are heating and cooling. Three newly developed systems for heating and cooling applications in buildings are proposed and assessed. Energy and exergy analyses are performed to assess the performance of heating, cooling and overall systems for each case, and the effects of various parameters on the energy and exergy efficiencies are examined. Also, the effect of changing the energy input for each system is also found in terms of overall efficiency. The overall system energy efficiency is found to be highest for the natural gas operated system with a vapour absorption chiller (system 1) at 27.5% and lowest for the photovoltaic (PV) and solar thermal operated system with vapour compression chiller (system 3) at 19.9%. The overall system exergy efficiency is found to be highest for the PV and solar thermal operated system with vapour compression chiller (system 3) at 3.9% and lowest for the PV and solar thermal operated system with heat pump (system 2) at 1.2%, respectively.

Performance and profitability perspectives of a CO2 based district energy network in Geneva's City Centre

A new type of district energy network providing simultaneously heating and cooling services is being investigated. It is based on the use of CO2 as a heat transfer fluid by taking advantage of the latent heat of vaporization, to store and transfer heat across the network. The goal of the present study is to determine the performance of a CO2 network when applied to a real urban area. It focuses first on determining the requirements for the various thermal energy services for a part of Geneva's City Centre. The final energy consumption is first computed for the energy conversion technologies now in place in this area – namely heating oil boilers and air cooled compression chillers. Then the new final energy consumption is computed considering that a CO2 network is used instead of boilers and air cooled compression chillers. For the area considered the CO2 network's variant leads to a final energy consumption of 10,968 MWh of electricity. It represents a reduction of 84.4% when compared to the boilers and chillers case. A comparative profitability analysis of the two cases is also presented. The analysis takes into account investment, heating oil and/or electricity, equipment replacement, operation, and maintenance costs, as well as the sales of energy services.For an interest rate of 6%, a price of the delivered heating/cooling services at 0.108 € kW h−1 and a lifetime of 40 years, the conventional technology does not reach profitability while the CO2 network achieves a profit in present value of 69.59 million € and the break-even point is reached after 5.5 years of operation.

Integration of an adsorption chiller in an open-cycle desiccant-assisted air-conditioning system

In a pilot installation at Hamburg University of Technology, the coupled operation of an open-cycle desiccant-assisted air-conditioning system with an adsorption chiller in combination with cooling ceilings to remove sensible loads is investigated. This article presents experimental results of the year 2013 with a special focus on the performance of the adsorption chiller. Furthermore, the system is compared to two reference systems relying on vapor compression chillers. The average thermal coefficient of performance of the adsorption chiller during the investigated period is satisfying (COPth = 0.54), while the average electrical coefficient of performance does not meet expectations (COPel = 6.2). The fan control of the dry cooler can be identified as a main cause for the non-satisfying electrical performance. However, depending on the heat supply, considerable primary energy savings are possible through the investigated system, and the advantages of combined heat and power (CHP) engines can be extended to the summer period.

흡수식 냉동기 재생기용 고성능 전열관의 풀비등 성능

흡수식 냉동기의 고효율화와 소형화를 위해서는 고성능 전열관의 연구가 필수적이다. 하지만 재생기용 전열관에 대한 연구는 많이 미흡한 실정이다. 본 연구에서는 만액식 재생기용 전열관의 성능을 파악하기 위한 기초연구로, 형상이 다른 7종의 전열관에 대해 풀비등 실험을 수행하고 비등열전달계수를 도출하였다. 실험은 압력 7.38~101.3 kPa, 열유속 $20{\sim}40kW/m^2$ 의 범위에서 수행되었다. 실험결과 모든 전열관에서 압력이 증가하고 열유속이 증가할수록 비등열전달계수 값은 증가하였다. 대기압 조건에서는 notched fin 관과 low fin 관이 높은 비등열전달계수를 나타내었고(열전달계수가 가장 낮은 19.0 mm O.D. 평활관의 225% 와 202%), 압력이 낮아질수록 low fin 관이 다른 전열관에 비해 현저히 높은 비등열전달계수를 나타내었다 (12.34와 7.38 kPa에서 19.0 mm O.D. 평활관의 290%와 288%). 【For performance improvement and compactness, usage of enhanced tube is inevitable. However, studies on enhanced tubes for generator is very limited. In this study, pool boiling tests were conducted for 7 heat transfer tubes. Test range covered pressure 7.38~101.3 kPa and heat flux $20{\sim}40kW/m^2$ . Results show that boiling heat transfer coefficient increases as pressure or heat flux increases. Under atmospheric condition, high heat transfer coefficients were obtained for notched fin and low fin tubes(225% and 202% of the 19.0 mm smooth tube, which yielded the lowest heat transfer coefficient). As pressure decreased, high heat transfer coefficients were obtained for a low fin tube(290% and 288% of the 19.0 mm smooth tube at 12.34 and 7.38 kPa).】

ENERGY EFFICIENT FOR REFRIGERATION AND AIR CONDITIONING

Over the years, all parts of a commercial refrigerator, such as the compressor, heat exchangers, refrigerant, and packaging, have been improved considerably due to the extensive research and development efforts carried out by academia and industry. However, the achieved and anticipated improvement in conventional refrigeration technology are incremental since this technology is already nearing its fundamentals limit of energy efficiency is described is ‘magnetic refrigeration’ which is an evolving cooling technology. The word ‘green’ designates more than a colour. It is a way of life, one that is becoming more and more common throughout the world. An interesting topic on ‘sustainable technologies for a greener world’ details about what each technology is and how it achieves green goals. Recently, conventional chillers using absorption technology consume energy for hot water generator but absorption chillers carry no energy saving. With the aim of providing a single point solution for this dual purpose application, a product is launched but can provide simultaneous chilling and heating using its vapour absorption technology with 40% saving in heating energy. Using energy efficiency and managing customer energy use has become an integral and valuable exercise. The reason for this is green technology helps to sustain life on earth. This not only applies to humans but to plants, animals and the rest of the ecosystem. Energy prices and consumption will always be on an upward trajectory. In fact, energy costs have steadily risen over last decade and are expected to carry on doing so as consumption grows.

Cost-effectiveness of multi-purpose solar thermal systems and comparison with PV-based heat pumps

Even under the powerful thrust of 20–20–20 measures, solar thermal systems are experiencing a slow-down in their development in most EU countries. One reason is their traditional confinement to sanitary hot water (SHW) production; another one is the growing competition with photovoltaic (PV) systems. In order to widen the use of solar thermal collectors, they should also be able to contribute to space heating and cooling and become Multi-Purpose Solar Thermal Systems (MPSTS). This paper addresses the issue of optimal sizing of MPSTS. The criterion adopted is based on maximization of Net Present Value and has been applied to some cities in Italy and in Pakistan with diverse climate conditions. Results show that optimal thermal collector areas per peak cooling demand (Ac/Pc) can be conveniently expressed as a function of Peak Heating to Cooling Ratio (Ph/Pc). Optimum Ac/Pc varies between 3 and 5m2/kWc and decreases with increasing Ph/Pc. The paper also analyses and compares a MPSTS with a multi-purpose PV-based heat pump system using a traditional one (boiler and compression chiller) as reference. Results show that steadily decreasing prices have made PV systems more favorable, even without consideration of public subsidies.

Energy and exergy analysis of subcooling the condenser outlet refrigerant in a compression-absorption cascade refrigeration system

The main aim of the presented research is to study the effect of subcooling the condenser outlet refrigerant in a compression-absorption cascade refrigeration system via energy and exergy methods. A parametric study is also conducted to investigate the effects of the operating conditions. LiBr-H2O and H2O-NH3 are used as fluid pairs in absorption section. The system is powered up by a prime mover, which can be either a micro-gas turbine, a gas turbine or a solid oxide fuel cell. The waste heat from the prime mover operates the absorption chiller and provides additional cooling, which is used to subcool the liquid exiting the condenser of the compression chiller. Results indicate that subcooling the condenser outlet enhances the efficiency of the cycle. It is also concluded that the system with a H2O-NH3 chiller has a better overall efficiency than the system with a LiBr-H2O chiller.

Feasibility Study of the Absorption Chiller Integration

Paper presents feasibility study of the absorption chiller integration. In the search model system of air conditioning uses absorption chiller instead of the standard vapor compression chiller due to the fact of insufficient electrical supply of the building. Calculated payback periods show limits of the effective applying of the absorption chiller.