ASHRAE Journal - March 2009 - (Page 54)

or COP). In cooling for example, higher 1.400 groundwater flows result in a smaller ΔT 1.200 on the groundwater, and a lower ground1.000 water exit temperature that permits lower temperatures on the building side of the 0.800 exchanger. Strictly from the building 0.600 loop performance perspective, higher 0.400 groundwater flows are always preferable 0.200 and accordingly, many systems have been designed with groundwater flows in the 0.000 53 80 106 159 212 265 310 424 500 2 to 3 gpm/ton (0.036 to 0.054 L/s · kW) System Capacity (tons) range. When one considers the perforClosed Loop Open Loop mance of the system however, including well pump power, as groundwater flow Figure 2: Relative capital costs—open and closed loop. is increased system performance (EER or COP depending on the mode) increases to a point where it Ground Surface reaches a maximum beyond which increases in groundwater Pitless Adaptor flow result in decreased system performance due to well pump Surface Seal (Bentonite or Cepower requirements. ment) Minimum 18 ft Typical. The key to open loop system design is to gather the necessary May Extend to Top of Injection Zone in Injection Well data to enable the type of calculation in which the sum of well Static Water Level pump, loop pump, and heat pump power (system performance) is evaluated over a range of groundwater flows. Key data required for this calculation includes production well pumping levels over a range of flows and heat pump performance over a range of entering water temperatures. The former is provided by the results of a well Drawdown Cone of Depression pump test and the later from manufacturer’s performance data. The first step is to calculate the well pump power over a range Pumping Water of flows. Table 1 provides a calculation for an example case. The Level pump head associated with the well (SWL + DD) is typically the Pump Column largest component of total well pump head in most applications. Pump “Bowl” Assembly In the absence of a pressure sustaining device (described later) Well Casing May the injection well in this example case would actually impose Pump Motor Equipped Be Installed Only a small “negative” head at low flow rates (<225 gpm [14 L/s]) With Cooling Shroud at Surface (Open Hole) or to Top as the injection water level would remain below ground level. of Production Summing the three components of the pump head and applying Zone Depending pump and motor efficiencies allows the determination of well on Geology Production Zone pump power requirements. Completion With Screen, Slotted Casing or Open Table 2 presents the results of combining the well pump power Hole Depending on with the building loop component power to arrive at system perforGeology mance. For this particular application the optimum peak groundwater flow for the cooling mode is approximately 1.6 gpm/ton Figure 3: Water well terminology. (0.027 L/s·kW). Designing the system for any flow in excess of this would result in greater cost, lower efficiency, possibly higher requirement of the secondary mode. Additional detail on pump regulatory scrutiny, and maintenance costs for the owner. control strategies are covered in the references.4,5 Over a fairly broad range of commonly encountered well pumping conditions the optimum groundwater flow falls into the Groundwater Handling range of 1.0 to 1.75 gpm/ton (0.018 to 0.031 L/s · kW)4 though One of the critical issues in the design of any system handling only through a calculation such as that in Tables 1 and 2 can the groundwater is to configure the design in a way to eliminate or optimum for a particular application be determined. Through a minimize problems arising from the groundwater chemistry. similar calculation optimum flow for the heating mode can be For GWHP systems, the following are the primary issues in determined as well. The well, pump, heat exchanger, and piping groundwater handling: are selected based on the larger of the cooling and heating mode • Verification of regulatory issues; optimum flows. Pump controls (variable speed or cycling) are • Early drilling and flow testing; then configured to allow the system to operate at the lower flow • Water chemistry analysis; 54 ASHRAE Journal Relative Ground Loop Cost ashrae.org March 2009 http://www.ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal - March 2009

Contents Commentary Industry News Letters Meetings and Shows Special Section 2009 ASHRAE Technology Awards Feature Articles Heat Recovery for Office Tower Air Motion Control in the Hospital Operating Room Anniversary Feature: Five Defrost Methods for Commercial Refrigeration Groundwater Issues: Commercial Open Loop Heat Pump Systems Used Filters and Indoor Air Quality Building Sciences Emerging Technologies Washington Report Products Special Products People Classified Advertising Advertising Index

ASHRAE Journal - March 2009

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.