| Title | Investigation of Ethanol Based Secondary Fluids with Denaturating Agents Used for Borehole Heat Exchangers |
|---|---|
| Authors | Monika IGNATOWICZ, José ACUÑA, Åke MELINDER, Björn PALM |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | secondary fluid, borehole heat exchanger, ground source heat pump, thermophysical properties, pressure drop, ethanol |
| Abstract | Ground source heat pumps are commonly used in Sweden. The dominating method of exchanging heat with the ground is by circulation of a secondary fluid through borehole heat exchangers inserted into energy wells. Due to the environmental reasons and relatively good thermophysical properties, aqueous solutions of ethanol are recommended and are commonly used for the borehole heat exchanger application in Sweden and other Nordic countries. The primary idea of using these fluids is to decrease the freezing point of the fluid since many borehole loops in Northern Europe operate near or below the freezing point of water. The available ethanol based commercial products consist of ethyl alcohol with up to 10% of denaturing agents without any corrosion inhibitors. The main denaturing agents used to prevent from drinking are mixtures of isopropyl alcohol, n-butanol, acetone, methyl ethyl ketone and methyl isobutyl ketone. These denaturing agents in commercial secondary fluids can influence the fluid’s thermophysical properties and thereby affect the pressure drop and heat transfer in the borehole heat exchanger. These not well defined effects of additives can cause uncertainties in the ground source heat pump design and operation. The thermophysical properties of a few aqueous ethanol solutions are experimentally studied in this paper, including a field sample, a pure mixture of ethanol, and ethanol including additives are compared with reference data. The density of 20 wt-% solution of ethanol including additives and the field sample were higher than pure 20 wt-% ethanol sample. The additives increased the specific heat capacity by 2.3% at -10 ºC and 1% at 0 ºC. A decrement by 10.5% in the dynamic viscosity was observed. Presence of additives increased the thermal conductivity values by around 1.5% at -8 ºC and 4.3% at +40 ºC. Some implications in terms of pressure drop and fluid to pipe thermal resistance are observed when these properties are applied in borehole heat exchangers, as the occurrence for laminar or turbulent varies at given temperature levels. An alternative would be to replace the denaturing agents with corrosion inhibitors to decrease the concentration of additives, improve corrosion protection of the systems and still fulfill the market recommendations. |