How does heat transfer fluid flow affect the efficiency of geothermal systems?
April 12, 2013 By Benjamin Hénault
You may doubt that the actual flow of heat transfer fluid in your
geothermal loop plays an important role in effective heat transfer
You may doubt that the actual flow of heat transfer fluid in your geothermal loop plays an important role in effective heat transfer. But did you know that its impact is either too great, or negligible? Did you know that without a flow regulator of some kind, flow within the loop will change constantly over the year? Flow is generated by a circulation pump (and its motor), and depends on the pump’s efficiency, power rating, and the system’s overall bleed rate. The bleed, or loss, rate will vary as a function of loop configuration (diameter, durability, length, pipe fitting) and fluid properties (viscosity, density, type, concentration). Fluid properties will vary as a function of fluid temperature. In summary, if a parameter changes, the balance of the system is undermined. Since the system is a loop, entering values depend on exiting values.
You likely have already heard of flow types, of which two concern us: laminar and turbulent.
Between these two modes, there is a transitory state, where fluid is sometimes laminar and sometimes turbulent. Each mode has its advantages. Laminar flow allows fluid circulation at much lower pumping energy than turbulent. Turbulent flow gives us a comparatively better heat exchange at each spot on the pipe, as the fluid’s thermal resistance is smaller. More precisely, the heat exchanger will be smaller, and implicitly less expensive, to extract the same quantity of heat from the same geology. A geothermal system running at full capacity must always have turbulent flow. In economic terms, a compromise is required in paying more for pumping energy to reduce the total HX length.
Geothermal system designers must ensure that the systems provide turbulent flow in the worst possible operating conditions, i.e., the lowest system temperature. The lower the slower the heat transfer fluid flow, the more viscous the fluid must be. The more viscous the fluid, the more the flow approaches laminar flow. The less heat exchange that occurs in this laminar mode, the more the heat transfer fluid chills.
A too-high antifreeze concentration in heat transfer fluids will raise fluid viscosity for the same temperature, even though its freeze protection will be inferior. There is also no need to have too high of a concentration in the loop, as doing so will raise flammability and handling safety concerns, and the system will run generally less efficiently. Compliance with the minimum freeze protection prescribed by CSA C-448 is sufficient.
Manufacturers recommend circulation flow to the heat pump. Generally, this figure is three U.S. gallons per minute, per ton of refrigerant. A simple system will have the same total circulation flow in its borehole field as in the heat pump. The more boreholes are connected in parallel, the smaller the flow in each borehole; thus the choice of pipe diameter matters therefore to borehole field flow type and rates. The same applies to pressure losses – they must be the same so that flow rates in all boreholes are the same. At first glance, a geothermal system’s operations are quite simple: a pipe in the ground that can capture or reject heat. However, for system cost-effectiveness, knowing which factors affect loop flow is fundamental to avoid surprises during system startup. Are you certain that your geothermal systems are working with optimal flows? Perhaps that may be a source of your problems.
Benjamin Hénault is a professional engineer. Since graduating from École de technologie supérieure, he has worked as the technical advisor at Canadian GeoExchange Coalition and is studying geothermal research for his masters degree at École Polytechnique de Montréal.
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