Ming Qu

A Linear Parabolic Trough Solar Collector Performance Model

A performance model has been programmed for a solar thermal collector based on a linear parabolic trough reflector focused on a coated absorber tube enclosed in an evacuated transparent tube: a Parabolic Trough Solar Collector (PTSC). This steady state, single dimensional model is based on fundamental material and energy balances together with heat transfer correlations programmed in the Engineering Equation Solver (EES). The model considers the effects of solar intensity, incident angle, collector dimensions, material properties, fluid properties, ambient conditions, and operating conditions on the performance of the PTSC. The model has been used to size system devices, to choose proper operating conditions, and to detect possible operating problems for the solar cooling and heating system for the Intelligent Workplace (IW) at Carnegie Mellon University (CMU) in Pittsburgh. The IW installed 52 - square meter PTSCs coupled with a 16 kW absorption chiller for space cooling and heating in August of 2006. The tests on PTSC performance are now being carried out. After the model is validated by experimental data of the tests, it will be further used to improve PTSC design and to optimize system operation and control for the IW.Copyright

Experimental and Model Based Performance Analysis of a Linear Parabolic Trough Solar Collector in a High Temperature Solar Cooling and Heating System

An innovative solar cooling and heating system has been designed, installed, tested, and modeled at Carnegie Mellon University to assess the technical and economic feasibility of high temperature solar cooling and heating system. This system primarily consists of parabolic trough solar collectors (PTSC) and a double effect absorption chiller. A comprehensive model for the tubular receiver of the PTSC has been developed to improve the PTSC design and overall system performance. The model has been verified by the experimental data from the tests on the PTSC in this system. The experimental data and theoretical analysis demonstrated that the properties of the glass envelope of PTSC significantly impacted the PTSCs performance. The model calculations indicated that the vacuum in the annular space between the glass tube and absorber pipe of the PTSC does not markedly improve its efficiency. In addition, the system performance of the high temperature solar cooling and heating system has been presented and evaluated by using experimental data. Based on these model calculations, the performance of the PTSC installed has been projected and measures to improve the PTSC design have been recommended.

Experiment Based Performance Analysis of a Solar Absorption Cooling and Heating System in Carnegie Mellon University

The center for building performance and diagnostic (CBPD) at Carnegie Mellon University has successfully designed, installed and tested a solar cooling and heating system to assess the feasibility of solar cooling for small scale commercial buildings or residential buildings with aspects of technology and energy efficiency. This solar cooling and heating system is primarily comprised of parabolic trough solar collectors, PTSC’s and a 16 kW dual energy source double effect (2E) absorption chiller. The 2E absorption chiller driven by PTSCs was tested to produce chilled water or hot water throughout a number of clear days in summer and winter. The analyses of the experimental data defined the system performance: the efficiency of the solar collector, the capacity and COP of the chiller, and the heat transfer coefficient of the heat recovery exchanger, by using a statistical approach, based on the energy balance equation. In the solar cooling tests during July 2007 in Pittsburgh, PA, the average efficiency of PTSCs was 35% when they were operated at about 155°C for driving the 2E absorption chiller and the chiller was able to provide 8 to 14 kW cooling with COP in the range 1.0 to 1.2; the overall system efficiency is in the range 0.35 to 0.41. In the near future, this solar absorption cooling and heating test system and its operation will be integrated with the cooling, heating and ventilation units for long term utilization.Copyright

Optimization Under Uncertainty: A Case Study of a Solar Absorption Cooling and Heating System for a Medium-Sized Office Building in Atlanta

Solar absorption cooling and heating (SACH) systems currently still stay at development and demonstration stage due to the nature of the complex system. It is critical for practitioners and engineers to have a correct and complete performance analyses and evaluation for SACH systems with respects of energy, economics, and environment. Optimization is necessarily involved to find the optimal system design by considering these three aspects. However, many assumptions made in the optimization are sensitive to the energy, economic, and environmental variations, and thus the optimization results will be affected. Therefore, the sensitivity and uncertainty analysis is important and necessary to make optimization robust.This paper uses a case study to explore the influence of the uncertainties on the SACH system optimization results. The case is a SACH system for a medium size office building in Atlanta. The one parameter at a time (OAT) sensitivity analysis method was applied firstly to determine the most sensitive inputs. Monte Carlo statistical method was utilized to generate the data sets for uncertainty analysis. The optimization problem under uncertainty was then formulated and solved. Due to the uncertainty associated with system inputs, the optimization solutions were found with certain types of the distributions. In addition, the scenario analysis on electricity price does not show large sensitivity to the objectives.Copyright

The Heat Transfer Characteristics of a 16 kW Steam Driven Double Effect Absorption Chiller

A 16 kW (4.6 refrigerant tons) steam driven, double effect, parallel flow absorption chiller has been designed, manufactured, and installed in the Intelligent Workplace (IW) of Carnegie Mellon University (CMU). This chiller is driven by 6 bar saturated steam and uses a 57% LiBr-H2 O sorbent. It is the smallest absorption chiller available in the existing market. The absorption chiller consists of five major and four minor heat transfer components. The manufacturer of the chiller has provided information on detailed configuration and dimensions of these components to support the calculation of their heat transfer areas, A’s, and the estimation of overall heat transfer coefficients, U’s. A steady state computational performance model for the chiller has been developed based on the applicable scientific and engineering principles. The model has been used to calculate all chiller internal working conditions and to analyze the experimental data over a wide range of operating conditions. Heat transfer coefficients inside and outside of the tubes making up the chiller’s heat transfer components have been estimated by published empirical correlations. The product of the overall heat transfer coefficient and the surface contact area, UA’s, for the 5 major heat transfer components have been estimated using the chiller model and measured performance data. Significant variations, 30%, in this parameter are observed under partial load, reduced flow conditions. Deviations between the experimental measurements and the model solutions have been analyzed to evaluate the model accuracy. At design operating conditions, the overall deviation is about 6%.Copyright

Effectiveness Model of Cooling Coil Unit Based on Parameters Identification

The performance of cooling coil unit (CCU) can directly influence the performance of ventilating and air conditioning (HVAC) system. In this paper, a dynamic CCU model was obtained by identifying the unknown parameters of existed effectiveness model. Five different conditions information is used to identify five model parameters by an optimal method. Unlike existed effectiveness model, the identified model can be simply determined by flow rate of chilled water, the temperature and humidity of return air and temperature of supply chilled water without requiring geometric specifications, which is very convenient in real engineering application. It was validated by five different experiment conditions on a CCU. The experiment results show that the identified model has a high accuracy despite changing the temperature and volume of chilled water.Copyright

Flow Resistance Coefficient Identification of Chilled Water System Based on Multi-Objective Optimization and Experiment Validation

Hydraulic resistance coefficient (HRC) is a fundamental parameter that characterizes the hydraulic state of a water pipeline and significantly determines the efficiency of the water-transport process. To estimate HRC and diagnose hydraulic process fault in building air conditioning system, a novel method called multi-objective optimization (MBO) strategy was developed in the research effort. MBO is concerned with mathematical optimization problems involving more than one objective function to be optimized simultaneously. In this paper, first, the basic principle of the approach is presented. Then several experiments are conducted to identify the HRC in a real air conditioning system. And the water flow rate of each air handling terminal unit is estimated by the flow rate of primary pipe and identified HRC. The experiment results show that the model can accurately estimate HRCs. The HRCs of each pipe and terminal unit were obtained by the flow rate and the pressure difference of primary pipe without requiring geometric specifications, which is very convenient in real engineering application.Copyright

Experimental Performance Analysis of External Compound Parabolic Concentrators With Low Concentration Ratios for Medium Temperature Applications

Due to the mounting concerns about climate changes and depletion of fossil fuels, solar energy, as one type of renewable energy, has attracted a lot of interests from academia, industries, and government in the past few decades. Currently, solar thermal technologies have been applied to the applications at the low operating temperature below 100°C by using flat-plate solar collectors and at the high operating temperature above 250°C by using solar tracking concentrators. For the medium operating temperatures between 100°C and 250°C, flat-plate solar collectors can hardly reach 100°C and solar tracking concentrators are too expensive. In this context, the use of external compound parabolic concentrators (XCPC) for applications operated at medium temperature draws quite attentions because of its higher efficiency than flat plate solar collectors and better cost effectiveness than solar tracking concentrators. However, currently only a few experimental data is available on the actual performance of XCPCs from literatures, especially for the recently new XCPCs with a low concentration ratio. In order to contribute to the knowledge, a series of experiments have been conducted on the new XCPCs recently installed at Bowen Lab, Purdue University, West Lafayette, Indiana. The experiments showed that the XCPCs raised the temperature to 170°C, which is 140°C higher than the ambient temperatures, with a thermal efficiency of 29%. Based on the data collected from the experiments, the optical and thermal efficiencies of XCPCs are determined for different solar irradiations, operating temperatures, and incident angles. A new regression model is proposed and fitted accordingly. The experimental data and analysis demonstrated the feasibility and potentials of using XCPCs for applications in medium temperature range such as solar absorption cooling and heating systems, seawater desalination, solar disinfection, post-combustion carbon capture systems and other industrial process heating.Copyright

Model Based Analysis of an Integrated System of Vapor-Compression Chiller and Absorption Heat Pump

Subcooling of the refrigerant at the exit of the condenser in a vapor compression refrigeration system could be an effective method to improve the coefficient of performance (COP). This method allows the refrigerant entering the evaporator with a lower mass fraction of vapor and absorbs more heat in the evaporator. The effort in this paper investigates a solar based integrated system of an electricity-driven vapor-compression chiller (VC) and an absorption heat pump (ABS) to provide both heating and cooling to space. Thermodynamic and heat transfer models of the integrated system were developed to estimate the system performance. The modeling results showed the integrated system can achieve 40% of the reduction on electricity consumption and 66% of the improvement in cooling COP. Furthermore, the hot water supply temperature can reach 50.25°C.The models have also been used to conduct parametric sensitivity analysis. The key parameters which affect the performance of the system were the heat source temperature, the hot water return temperature, and the outdoor air flow rate. Hot water flow rate only has significant influence on the hot water supply temperature. Increasing the temperature and flow rate of the heat source can have benefits on both heating and cooling performance. However, increasing the outdoor air flow rate can only benefit on energy saving and cooling performance.Copyright

Compound Parabolic Concentrators in Solar Thermal Applications: A Review

Among all types of concentrators, compound parabolic concentrators (CPCs) have been designed as stationary solar collectors for relative high temperature operations with high cost effectiveness. The CPCs are potentially the favorable option for solar power systems and high temperature solar thermal system. This paper provided a review on studies of CPCs in solar thermal applications. It covered basic concepts, principles, and design of CPCs. It also reviewed optical models and thermal models of CPCs, as well as the thermal applications of CPCs. The challenges were also summarized.Copyright