Water Well Journal
Issue link: http://read.dmtmag.com/i/465664
L ast month we kicked off a two-part series on a geother- mal water supply system using a groundwater source for a local housing development with an introduction on the principles of geothermal sources, a background on the devel- opment itself, and an overview of the water well planned for use. This month we will outline the design concepts and con- struction methods that were implemented. But as opposed to a typical column, this article has been formatted in a quasi- specification format to aid the reader with an understanding of the design concepts used. Design-Build Project Delivery In order to expedite the design and construction phases of the project, the developers opted to make use of the "design- build" method of project delivery. This method ensured sole source responsibility for the entire system as well as acceler- ated the final completion through a carefully coordinated design and construction procedure. Although the basic principle of using groundwater for heat extraction or cooling is fairly routine, the developers desired to use the water to the fullest possible potential. Therefore, the decision was made to incorporate on-grounds irrigation into the system. This required the use of higher pressures than would typically be needed. During the initial design process, consideration was given toward the best method of wastewater disposal. The three primary methods evaluated were: 1. Surface water disposal via Pringle Creek, which ran through the development site 2. Water well disposal, either through a new recovery well or back into the source well 3. A closed loop system where the water would be circulated constantly through the system and temperature stabilization accomplished through the use of make-up water from the well. Due to water rights and wastewater disposal limitations, the use of surface water sites was quickly discounted. Imple- mentation of a closed loop system, while feasible, involved too much complexity in the controls and created a problem for the required parallel use of on-grounds irrigation. Finally, computer modeling of the projected temperature shifts within the system indicated disposal of the recovery water back down the same production well would be difficult to modulate as the aquifer did not contain sufficient stratifica- tion in which to provide the needed balance. System Description The configuration of the system is shown in the geothermal water distribution system diagram (Figure 1). An existing well, described in Part 1 last month, serves as the source of water for the system. This well is referred to as the source well, extraction well, or production well. Water is pumped from this well and is distributed through a supply piping system located in the streets to buildings and homes throughout the community. Two new submersible well pumps are provided to meet the requirements of the geothermal water distribution system. The smaller of the two pumps, referred to as the lead pump, is a 7.5 hp submersible pump designed to fulfill the system flow requirements from 5 up to 100 gpm. The second pump, the lag pump, is a 20 hp reserve and high-demand pump for the water demands between 100 up to 250 gpm and as a backup to the lead pump. At each building or home, the water is circulated through heat pumps, where heat is extracted from the water (heating mode), or rejected to the water (cooling mode). The projected range of flows and pressure loss through a typical heat pump system is 1.5–6 gpm with a pressure loss between 3 psi to a maximum of 10 psi. Maximum heat gain or loss is projected to be as much as –9°F (heating mode) to a high of +23°F (cooling mode). At a typical well water temperature of 53°F, this equates to a range of injection water temperature between 44°F during peak heating loads to a high of 76°F during the maximum pro- jected cooling loads. Due to the expected intermittent opera- tion of heat pump units and intersystem mixing, the actual range is expected to generally range between 48°F to 62°F. After passing through the heat pumps, the water leaves the building and enters a return piping system located in the street. Once in the return piping system, the water is available ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS GEOTHERMAL SUPPLY AND RECOVERY SYSTEM USING A GROUNDWATER SOURCE Design concepts and construction methods: Part 2 ENGINEERING continues on page 38 36 March 2015 WWJ waterwelljournal.com Temperature sensors, water level transmitters, and flow-meters at both source well and injection wells are used for monitoring of operation and for educational purposes.