Reducing the economic costs of a direct-use geothermal heating system

Direct use geothermal systems use groundwater that is heated by natural geological processes below the Earth’s surface. This water can exceed 200°F (over 90°C). Such hot groundwater bodies can be found in many areas with volcanic or tectonic activity. There is a great potential for geothermal energy exploitation, but the high economic costs associated with well drilling and heat exchanger maintenance limit the use of this technology for space heating. Namely, metallic heat exchangers require great purchase and maintenance costs due to the chemical aggressiveness of the geothermal fluid.

 

A team of Italian researchers has modelled a plastic plate heat exchanger for a direct geothermal heating system. For this type of application, polymer heat exchangers have a lower purchase cost and a better resistance to fouling than metal heat exchangers. The plastic plate heat exchanger was modelled geometrically and thermodynamically, and then optimised using an exergoeconomic analysis. The proposed model was applied to a case study in southern Italy, in an active volcano district of the Campania region characterised by high geothermal fluid temperature (in the range 90–120°C) at shallow depth of 86–101 m. The plate heat exchanger was used to meet the heating needs of a single building.

 

The researchers obtained the following results:

• the overall heat transfer coefficient presented the greatest values for a high temperature of geothermal fluid (105–120 °C) and for a number of channels from 7 to 12 for each flow variable.
• the required heat exchanger surface areas were low (and consequently, the purchase cost of the heat exchanger) for an overall heat transfer coefficient equal to about 240–250 W/K·m².
• the investment cost of the heat exchanger decreased when the geothermal inlet temperature increased. On the contrary, the investment cost of the well and the cost of electricity increased with temperature.
• the variation of the exergoeconomic costs of the well and electricity was lower than that of the heat exchanger; thus, the exergoeconomic cost of the product showed a trend similar to heat exchanger cost.

 

The exergoeconomic cost of the product was lowest when the temperature of the geothermal fluid was higher than 105°C; thus, the smallest heat exchange area was required. The optimum solution found using exergoeconomic optimisation, was a total product cost of 922€/y for a geothermal fluid temperature equal to 117 °C and with a number of plates equal to 15.

 

 

The study has been published in Energies.

image source: the Phlegrean Fields volcanic area in the south of Italy.