Cooling Load Demand Research Articles

A new solution for loading optimization of multi-chiller systems by general algebraic modeling system

In hot and humid regions, due to the limitation of peak power, it is vital to reduce the electrical power consumption of multi-chiller systems since their consumption noticeably increases if the chillers are improperly managed. For this aim, optimal chiller loading (OCL) is one of the most important issues in such systems. The ultimate aim of the OCL problem is to determine the partial load ratio (PLR) of each chiller with respect to the cooling load demand. Owing to the nonlinearity of the OCL problem, a powerful search method should be employed to efficiently solve this problem. In this paper, general algebraic modeling system (GAMS) has been proposed to solve OCL problem. GAMS is an efficient and easy to implement high level computer programming language for modeling and solving optimization problems (linear, nonlinear and mixed integer). In order to evaluate the efficiency of the proposed methodology, two case studies are solved and the results are compared with the results obtained by both conventional and heuristic search methods. The results represent that GAMS produces promising results in comparison with the other methods.

Hourly Performance Prediction of Solar Ejector-Absorption Refrigeration Based on Exergy and Exergoeconomic Concept

Solar cooling technology is being increasingly studied due to its environmental compatibility and cost saving capability. In this article, application of solar absorption refrigeration for cooling of an office building is investigated through modeling and simulation. Solar radiation and cooling load demand have hourly modeled during summer days using Tehran’s climate data as a typical weather scenario. The simulation results show that from 9 AM to 5 PM, product cost per exergy unit ( c P,tot ) for the entire system decreases 87%. During this time, thermodynamic coefficient of performance ( COP th ) increases from 0.16 to 0.48 (auxiliary heat needed reduces from 4.36 to 1.23 kW), suggesting that the performance of the system increases until 5 PM. This is also confirmed by studying exergetic coefficient of performance ( COP exe ) which reaches to the maximum point at the same time. Furthermore summer days analysis shows that at maximum air temperature the system has optimal COPs while the minimum of c P,tot occurs at maximum radiation. The results show the exact times at which the system performs less efficiently by solar energy and therefore the opportunity of using other available renewable energy resources at these times.

Effects of vegetation, urban density, building height, and atmospheric conditions on local temperatures and thermal comfort

This paper shows the effects of several variables, which co-cause the Urban Heat Island effect on temperature distribution and outdoor thermal comfort (by using the Predicted Mean Vote, PMV) on dense urban environments. The study was conducted by means of a three-dimensional microclimate model, ENVI-met 3.1, which forecasts the microclimatic changes within urban environments. The effects of building density (% of built area) and canyon effect (building height) on potential temperature, mean radiant temperature, and Predicted Mean Vote distribution are quantified. The influence of several types of green areas (vegetation on the ground and on roofs) on temperature mitigation and on comfort improvements is investigated for different atmospheric conditions and latitudes in a Mediterranean climate. The research quantifies the effects of the variables investigated on temperature distributions and in determining outdoor comfort conditions. Vegetation on the ground and on roofs mitigates summer temperatures, decreases the indoor cooling load demand, and improves outdoor comfort. The results of the study demonstrate that density and height of buildings in a city area influence potential temperature, mean radiant temperature, and Predicted Mean Vote distribution; for most of the cases examined higher density causes higher temperatures and with taller buildings vegetation has higher cooling effects. Considering the cooling effect of vegetation, a difference can be noticed depending on the amount of green areas and vegetation type. The results of this study show also that vegetation is more effective with higher temperatures and lower relative humidity values in mitigating potential temperatures, mean radiant temperatures, and PMV and in decreasing the cooling load demand.

Model-based optimization for vapor compression refrigeration cycle

This paper presents a model-based optimization strategy for vapor compression refrigeration cycle. Through analyzing each component characteristics and interactions within the cycle, the optimization problem is formulated as minimizing the total operating cost of the energy consuming devices subject to the constraints of mechanical limitations, component interactions, environment conditions and cooling load demands. A MGA (modified genetic algorithm) together with a solution strategy for a group of nonlinear equations is proposed to obtain optimal set point under different operating conditions. Simulation studies are conducted to compare the proposed method with traditional on–off control strategy to evaluate its performance. Experiment results of a real practical system are also presented to demonstrate its feasibility.

Historical Data Based Models for Chilled Water Production from Waste Heat of Turbine

The objective of this study is to model chilled water production from waste heat of a cogeneration plant using absorption process. Regression method is used to capture the relationship between the input and output parameters. The models for cooling load demands are addressed on hourly, daily and monthly basis, respectively. The methods are tested by considering two 12Ton/hr heat recovery steam generators, one 6Ton/hr auxiliary gas boiler, and two 1250RT steam absorption chillers. It was observed that the proposed method is effective in providing better picture of the relationships between the supplied heat and the amount of energy recovered by each system. Calculated using eight years data, the maximum cooling demand occurred in August and is about 2392RT. The steam generators were found operating at part load ratio of about 0.41 only. Both chillers deteriorated in performance within the study period. While the absorption chillers run mostly at a part load ratio of about 0.8 and consumed 43% more steam than required compared to design. The developed generalized characteristics are R2 = 0.9996 , c 2 = 1.9765e - 5 , and RMSE = 0.0044 .

Decentralized optimization for vapor compression refrigeration cycle

This paper presents a model based decentralized optimization method for vapor compression refrigeration cycle (VCC). The overall system optimization problem is formulated and separated into minimizing the energy consumption of three interactive individual subsystems subject to the constraints of hybrid model, mechanical limitations, component interactions, environment conditions and cooling load demands. Decentralized optimization method from game theory is modified and applied to VCC optimization to obtain the Perato optimal solution under different working conditions. Simulation and experiment results comparing with traditional on–off control and genetic algorithm are provided to show the satisfactory prediction accuracy and practical energy saving effect of the proposed method. For the working hours, its computation time is steeply reduced to 1% of global optimization algorithm with consuming only 1.05% more energy consumption.

Thermo-economic optimization of the impact of renewable generators on poly-generation smart-grids including hot thermal storage

In this paper, the impact of not controllable renewable energy generators (wind turbines and solar photovoltaic panels) on the thermo-economic optimum performance of poly-generation smart grids is investigated using an original time dependent hierarchical approach.The grid used for the analysis is the one installed at the University of Genoa for research activities. It is based on different prime movers: (i) 100kWe micro gas turbine, (ii) 20kWe internal combustion engine powered by gases to produce both electrical and thermal (hot water) energy and (iii) a 100kWth adsorption chiller to produce cooling (cold water) energy. The grid includes thermal storage tanks to manage the thermal demand load during the year. The plant under analysis is also equipped with two renewable non-controllable generators: a small size wind turbine and photovoltaic solar panels.The size and the management of the system studied in this work have been optimized, in order to minimize both capital and variable costs. A time-dependent thermo-economic hierarchical approach developed by the authors has been used, considering the time-dependent electrical, thermal and cooling load demands during the year as problem constraints.The results are presented and discussed in depth and show the strong interaction between fossil and renewable resources, and the importance of an appropriate storage system to optimize the RES impact taking into account the multiproduct character of the grid under investigation.

Analysis of Solar Powered Air Conditioning Systems for Residential Applications

In this paper the absorption air conditioning system has been analyzed, and the solar energy has been used as a source for heat required to operating the absorption cycle. The influence of most of the parameters which may affect the performance of absorption air conditioning system has been studied under the climatic conditions of Iraq. The main advantage of solar based air conditioning systems is the synchronization between the cooling demand and the availability of solar energy especially in hotregions like Iraq which containshuge potential of solar energy in summer season. The results obtained indicate the ability of using solar energy to reduce the electricity consumption for air conditioning applications. Fora certain space with cooling load 5 Ton (17.5 kW) for climatic conditions (solar radiation intensity 1000 W/m2 and ambient temperature 45oC) the solar collector with area 40 m2 can provide 75% of required heat to produce this cooling load demand and a collector with area53 m2 can provide 100 % of required heat to produce this cooling load demand for the seclimatic conditions.

The Impact of Airtightness on Energy Conservation of Conventional Cypriot Detached Houses

It is well acknowledged that construction industry suffers from drawbacks of poor construction implementation. Airtightness for instance, is one of the prominent factors which contribute to excessive building energy consumption, in the operation phase of the project. The aim of this research is to demonstrate the disadvantages of low-quality construction implementation from the airtightness point of view, in the early design stage by integrating Building Information Modeling and thermal performance modeling, taking conventional detached single-family dwellings in Cyprus as the case study. Different rates of air change per hour in the insulated and uninsulated residences were considered and, as a result annual energy consumption of an insulated residence which ACH rate was 2.5 - that accounts for poor implementation - computed to be roughly the same as a similar uninsulated case which ACH rate was 0.25 (29942 kWh compared to 30075 kWh). In addition, cooling load demand remained steady at approximately 25000 and 20000 kWh for uninsulated and insulated cases respectively, under the effect of altering ACH rate.

Feasibility of thermal energy storage systems in an institutional building in subtropical climates in Australia

Thermal energy storage is (TES) a preferred demand side management (DSM) technology for shifting cooling load demand from peak hour to off-peak hour in the heating, ventilating and air conditioning (HVAC) industry. In this study, the technical and economical feasibility of introducing TES systems in a building in subtropical Central Queensland (Australia) is presented. Firstly, the cooling load profile of existing systems is simulated using building simulation software DesignBuilder (DB) and verified with on-site measured data. Then, using the verified simulation technique, the technical and economical feasibility of TES systems are analysed for both full and partial storage scenarios. The results show that the full and partial chilled storage systems can save up to 61.19% and 50.26% respectively of the electricity cost required for cooling when compared with the conventional system in subtropical climate.

Solar hybrid cooling system for high-tech offices in subtropical climate – Radiant cooling by absorption refrigeration and desiccant dehumidification

A solar hybrid cooling design is proposed for high cooling load demand in hot and humid climate. For the typical building cooling load, the system can handle the zone cooling load (mainly sensible) by radiant cooling with the chilled water from absorption refrigeration, while the ventilation load (largely latent) by desiccant dehumidification. This hybrid system utilizes solar energy for driving the absorption chiller and regenerating the desiccant wheel. Since a high chilled water temperature generated from the absorption chiller is not effective to handle the required latent load, desiccant dehumidification is therefore involved. It is an integration of radiant cooling, absorption refrigeration and desiccant dehumidification, which are powered up by solar energy. In this study, the application potential of the solar hybrid cooling system was evaluated for the high-tech offices in the subtropical climate through dynamic simulation. The high-tech offices are featured with relatively high internal sensible heat gains due to the intensive office electric equipment. The key performance indicators included the solar fraction and the primary energy consumption. Comparative study was also carried out for the solar hybrid cooling system using two common types of chilled ceilings, the passive chilled beams and active chilled beams. It was found that the solar hybrid cooling system was technically feasible for the applications of relatively higher cooling load demand. The annual primary energy consumption of the solar hybrid cooling system was lower than that of the conventional vapour compression refrigeration system up to 36.5%. Between the two options of chilled ceilings, the passive chilled beams were more energy-efficient to work with the solar hybrid cooling system in the hot and humid climate. Harnessing solar energy for driving air-conditioning would help in reducing the carbon emission, hence alleviating the climate change.

Thermoeconomic assessment of a multi-engine, multi-heat-pump CCHP (combined cooling, heating and power generation) system – A case study

Design and operation of complex systems for combined cooling, heating and power generation (CCHP) are always a matter of matching performance and demand characteristics of a thermal system set to supply electrical, cooling and heating loads, according to specific usage demands. Equipment selection and operation require the characterization of power, heating and cooling load demands, and their time variation during years, seasons, months and even hours or minutes. The paper aims at utilizing a general model for complex CCHP systems. The proposed model is based on the general theory of exergy cost and structural coefficients of internal links. A general model is presented, and a simple hypothetical cogeneration case is studied. The system operates with two heat engines, with waste heat recovery driving a chiller, in order to meet electrical power and refrigeration loads.

HVAC system optimization––condenser water loop

This paper presents a model-based optimization strategy for the condenser water loop of centralized heating, ventilation and air conditioning (HVAC) systems. Through analyzing each component characteristics and interactions within and between cooling towers and chillers, the optimization problem is formulated as that of minimizing the total operating cost of all energy consuming devices with mechanical limitations, component interactions, outdoor environment and indoor cooling load demands as constraints. A modified genetic algorithm for this particular problem is proposed to obtain the optimal set points of the process. Simulations and experimental results on a centralized HVAC pilot plant show that the operating cost of the condenser water loop can be substantially reduced compared with conventional operation strategies.

Effect of refrigerant charge on the performance of air conditioning systems

Efficient management of energy in buildings necessitates the accurate evaluation of the air conditioning (AC) system performance. Ideally, the system must be operated with its cooling capacity matching the cooling load demand, and with maximum possible performance. This paper presents a simplified method that can predict cooling capacity in operative AC installations with limited input information. It is developed using an artificial neural network (ANN) with Bayesian regularization. The training data are generated by numerical simulations of operating scenarios representing the real system operation. The refrigerant temperatures at the inlet and outlet of the evaporator and the condenser are selected as inputs for the proposed method to predict the cooling capacity in relevant operating scenarios, thereby eliminating the need for a flow meter and facilitating implementation on operative systems. The ANN model is developed to capture the performance of different systems by using a data normalization method. The ANN prediction is tested on both simulation scenarios and experimental data with different nominal capacities. The results show that the ANN model can successfully predict cooling capacity variations using limited input parameters with RMSE and ΔQ-e,rel 0.09 kW and 3.99%, respectively.

Energy impact of commercial-building envelopes in the sub-tropical climate

Existing commercial buildings are surveyed and categorized based on the construction characteristics of their envelope. The envelope heat gain and the resulting cooling load demand are analysed with the aid of energy simulation tool DOE-2.1D. The concept of the overall thermal transfer value (OTTV) is applied to study the association of the envelope designs with the cooling requirement, and a modified approach in assessing the effective envelope heat gains under a sub-tropical climate is proposed. The predicted OTTV gives a good indication of the thermal performance of the envelope under a sub-tropical climate. The energy impact and possible range of cooling-load demands, for various envelope designs, under similar internal characteristics and cooling system, are identified.

Greenhouse Climate Control: an integrated approach

Controlling a greenhouse microclimate requires energy consumption for heating and cooling the greenhouse inside air. The radiative heat transfer process inside a greenhouse depends on the spectral radiative properties of different components especially on the greenhouse covering material. There is a lack of information about the precise spectral radiative properties of the available commercial plastic sheets used as greenhouse covers. In this research, the spectral radiative properties of several commercial greenhouse covers are measured in a wide wavelength range from 0.2 to 28 μm. The measured properties are then used in a developed model to simulate the indoor climate of a greenhouse. In the developed model, the covering material is treated as a semi-transparent medium, the soil surface is considered as partially reflective surface, and the inside air is assumed as an absorbing-emitting medium. Finally, under a specific hourly climate conditions, the impacts of using different commercial covers on heating and cooling load demands are investigated. The measured total solar transmittance was 70.3–92.1% and the long wave transmittance was 18.1–61.8% for selected samples. The results indicated that using the appropriate covering plastic reduced the annual cooling load by 9.8% and heating load by 6.3%.

Cooling load reduction of buildings using passive roof options

A model is presented to predict the thermal performance of a building with shading and roof ponds. A computer program is written to calculate hourly cooling load requirements by the numerical solution of the energy balance equation for the building. This simulation is validated for a bare roof by comparison with field data taken from an actual house in Shiraz, Iran. The effectiveness of the different roof options for passive cooling have been examined. Results indicate that for the house under study (of popular size and building material for Shiraz) cooling load demand reductions of 79.0%, 58.1% and 43.6% may be obtained by using shaded-pond, pond, and shaded roofs respectively.