Consulting-Specifying Engineer - November 2008 - (Page 36)

To terminal units (fan coil, radiant, etc.) Maximum 4 pipe dia. Cool Differential pressure transducer VFD Chilled water supply B Chilled water return A C Hot water supply Supplemental source Heat Differential pressure transducer VFD B A C Hot water return Heat pump Chilled water tank Heat pump Heat pump Hot water tank To bore hole field (individually or common piped) Figure 2 “Charging loop” configuration for smaller commercial buildings. Source: Peter D’Antonio, PCD Engineering Services Hybrid systems Hybrid systems (see Figures 1a and 2) use the heat pump system to meet a portion of the design load, and a majority of the annual load, thereby offering an opportunity to minimize initial costs and borefield requirements. Sizing the ground heat exchanger for the heating requirements of the building and adding a fluid cooler to supplement the heat rejection can minimize first cost of the system. This helps preserve the economics of the GSHP system. The use of a fluid cooler adds to the complexity of system controls and adds a piece of outdoor equipment, along with water treatment considerations. However, the potential cost reduction of the ground heat exchanger may justify this option in projects with cooling dominant loads. A hybrid system is generally attractive in installations where ground loop costs per ton are high, and where the heating loop length requirement is low (roughly 40%) relative to the cooling loop length requirement. Similarly, if the heating load greatly exceeds the cooling load, it is often more economical to meet a portion of the heating load with a boiler and size the loop for a smaller peak heating load. Maximizing benefits GSHP system benefits are greatest in buildings with similarly sized annual heating and cooling loads and those desiring independent climate control of many rooms with the potential for heating and cooling different zones simultaneously. Other good applications for the technology are facilities with relatively high occupancy, fluctuating usage schedules, and widely varying heating and cooling requirements within individual zones, such as offices and classrooms. GSHP system benefits are also greatest when a new building is being planned, or when considering the replacement of an existing system that no longer meets the needs of the building or has reached the end of its useful life. The design process Detailed design information is available from ASHRAE1; however, the general steps of the design process are as follows: 1. Ensure that the building is efficient as possible. Harvest passive energy sources, using high-quality insulations, glazing, and other envelope features to minimize loads and reduce air infiltration. This can be accomplished through retrofits or more cost-effectively during new building design. Mechanical engineers should work with the design team to stress the importance of reducing building load and evaluate the load reduction’s effect on overall mechanical system size and cost. 2. Calculate the building’s heating and cooling design loads. These calculations can be done manually or with a computer loads program. It is important to use accurate input data for internal gains, weather conditions, and building data to ensure that heating and cooling loads are not overestimated, which can result in oversized systems and drive up cost. Ensure the sizing takes into account the peak diversified or block loads to avoid oversizing and keep away from relying on rule-of-thumb sizing. As building system designs improve, old rule-of-thumb sizing becomes less accurate. 36 Consulting-Specifying Engineer • NOVEMBER 2008

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Consulting-Specifying Engineer - November 2008 Contents Viewpoint News M/E Roundtable Comissioning Lighting Controls Prevent Plugholing: Smoke Control Done Right Commercial Geothermal Heating and Cooling System Design Codes &�Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - November 2008

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