Open-loop, lake-source geothermal systems offer a sustainable alternative to conventional mechanical cooling by leveraging the relatively stable thermal properties of large natural waterbodies. However, the performance of such systems is highly sensitive to intake location, depth, and surrounding infrastructure - variables for which industry design standards such as the ASHRAE Handbook provide limited prescriptive guidance. This study investigates these variables through the case of Discovery World, a 120,000-square-foot museum in Milwaukee, Wisconsin, that operates a geothermal cooling-only system drawing approximately two million gallons per day directly from Lake Michigan. The facility reported summertime intake temperatures exceeding the 75°F heat exchanger design maximum, raising concerns about system efficiency and elevated utility costs.
To characterize near-shore lake temperature conditions, two HOBO MX2201 temperature logger lines were deployed in Lake Michigan adjacent to Discovery World from September 28 through October 5, 2025. Dock - Line 1 was positioned approximately 85 feet (ft) from the building near the existing intake, and Pier - Line 2 was deployed approximately 400 ft offshore at a candidate relocation site. These datasets were supplemented by historical temperature records from an EPA monitoring buoy and engineer-provided intake trend data from the WE Energies Oak Creek Power Plant, located 17 miles south of Discovery World. Lakeside flow readings were collected before and after heat exchanger cleaning to assess the performance impact of biological and sediment fouling.
The results tell a clear story. Lake temperatures at both monitoring locations stayed within a narrow range, between 66 and 71°F, well below the 75°F design limit, and the difference between the current intake and the candidate relocation site was negligible. This means moving the intake is unlikely to fix the performance issues on its own. The more telling finding came from flow testing, after the heat exchanger was cleaned. Flow improved by over 1,100 GPM, a strong indication that mussel buildup and sediment fouling are the real culprits. The most practical path forward is proactive maintenance: more frequent heat exchanger cleaning, water treatment to control mussel growth, and flow monitoring on the building loop. Ultimately, this study adds real field measurements to a corner of engineering practice that has long lacked them.
