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PERFORMANCE MODELING OF A SOLAR THERMAL SYSTEM FOR COOLING AND HEATING

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Copyright © 2007 by CBPD 1 Proceedings of ES2007 Energy Sustainability 2007 June 27-20, 2007, Long Beach, California ES2007-36053 PERFORMANCE MODELING OF A SOLAR THERMAL SYSTEM FOR COOLING AND HEATING IN CARNEGIE MELLON UNIVERSITY’S INTELLIGENT WORKPLACE Sophie V. Masson Research Engineer Ming Qu PhD. Candidate David H. Archer Professor of Engineering School of Architecture, Center for Building Performance and Diagnostics Carnegie Mellon University, Pittsburgh, PA 15213 ABSTRACT The Robert L. Preger Intelligent Workplace (IW) is a 650 m2 living laboratory of office space at Carnegie Mellon University (Pittsburgh, PA). The IW has received the first commercially available solar absorption system for air-conditioning with integrated controls as a donation from BROAD in August 2006. The IW is now testing this solar thermal system. A TRNSYS model has been developed and used to assist the design of the system, evaluate its performance throughout an entire year, and optimize its initial configuration. The components of the system are a 52 m2 parabolic trough high temperature solar array, a 16 kW hot water and gas fired absorption chiller, and an overall control system. This model predicts the energy required to cool and heat the south part of the IW (around 10 MWh in winter, 15 MWh in summer) and the fraction of that energy that can be provided by solar energy. The effects of significant system parameters – orientation of the receivers, volume of hot/chilled water thermal storage and insulation thicknesses on the piping and tank – on the fraction of solar provided energy have been calculated by the model. This study emphasizes on two significant aspects: - the impact of system integration during the preliminary building design on the energy performance - the importance of the energy modeling to assist and optimize the design of the system and its operation but also to reduce the investment and operation costs. 1. INTRODUCTION The world demand of energy for air conditioning is continually increasing. As traditional cooling devices are electrically powered, demand for electrical power in summer keeps increasing and reaches the capacity limit in some countries. Because most of the electrical power stems from fossil fired power plants this trend also increases the emission of CO2. A more innovative approach to provide cooling is to use solar energy in a heat driven active absorption cycle for air conditioning. The high correlation between the availability of solar energy and the need for cooling in a building provides an advantage to solar driven cooling. In addition, absorption based systems have the advantage of using reject heat from power generation as well as solar heat. A cooling system for building through absorption refrigeration makes direct and efficient use of solar heat, replacing the use of natural gas or electrical energy for vapor compression refrigeration. Approximately 50 to 60 % of the radiant energy impinging on the receivers is passed to the heat transfer medium for use. The coefficient of performance of absorption chiller, the fraction of heat removed in cooling to the heat supplied to the system is 1.1 to 1.2 for a two stage chiller. The performance of a solar driven absorption cooling system was studied and modeled in TRNSYS for an office space in Pittsburgh, PA, the Intelligent Workplace. The location of the IW is not an ideal location for solar application due to its latitude and frequently cloudy weather. Moreover the small scale of the solar driven cooling system militates against an economic application of the technology. Nonetheless, the TRNSYS model and the simulation evaluation can explore how solar energy through high temperature solar receivers and heat storage might be integrated into efficient energy supply system for a building, even in a place like Pittsburgh (latitude 40.3°N, longitude 79.6°W). 1.1 Building description The IW is a 650 m2 living laboratory of office space at Carnegie Mellon University. The IW south zone’s net floor area is about 245 m2. The average height is about 3.1 m, including the raised floor and average height of roof. The open space is subdivided by partition walls and furniture in 10 office or conference spaces. The building has horizontal shadings on the east and west facades. The IWs floor plan is shown in Fig.1.Copyright © 2007 by CBPD 2 Figure 1. IW south Floor Plan 1.2 Internal Loads The IWs is occupied throughout the by faculty and students. The internal loads in the IWs including lighting, plug loads such as computers, and occupants contribute to the overall cooling requirements. The maximum occupancy is 30 people. Most of the occupants arrive between 9AM and 10AM and leave the building between 5PM and 8PM. The equipment heat gain is 100 W per computer (one per person) and 50 W for one printer. The total heat gain of the artificial lighting is 17 W/m2. The equipment heat gain is based on the occupancy schedule and the lighting heat gain is based on solar radiation available. The lights are seldom turned on due to the architectural integration of day lighting features (skylight, windows) of the space. 1.3 HVAC System Ventilation and air conditioning are split to reduce the amount of air circulated: ventilation handles the latent and sensible loads of the outside air supply whereas air conditioning handles the sensible load of the space. 1.3.1 Ventilation. Fresh air is supplied to the IWs by a desiccant wheel heat pump based ventilation unit supplied by SEMCO via diffusers in the floor. The outside air volume rate is about 34 m3/hr per person based on the ASHRAE standards. The outside air is supplied during the whole year with -3°C to -6°C below the set room temperature at a humidity that maintains comfortable room conditions. 1.3.2 Air Conditioning. System design and modeling were performed before the procurement of the equipment (solar troughs, hot water and natural gas driven two stage absorption chiller, heat exchanger and storage tank). In a solar thermal system


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