Geothermal Loop and Heating Grid Interfaced by the Geothermal Heat Exchanger

 The thirty-four geothermal doublets (and as many heating grids), operating since the early 1980’s in the Paris area, totalise installed power and generating capacities of 230 MWt and 1,000 GWht/yr respectively and serve over 100,000 equivalent dwellings, each 70 m2 in area. They achieve the savings of 500,000 tons of CO2 emissions. Oradea, in Western Romania, is an example of the insertion of a geothermal heating system into the existing city, coal fired/back pressure, combined heat and power (CHP) network, typical of previous Central/Eastern Europe district heating practice. Eleven geothermal wells (2500-3450 m; 72-106 °C), among which two doublet arrays, are serviced for heat and sanitary hot water -SHW - supply amounting to ca 100,000 MWht/yr, via the CHP grid substations.

Worth recalling is that a GDH system has to comply with variable heat loads and existing building designs and heating modes. These conditions become acute for low outdoor temperatures (peak loads) and conventional, temperature demanding, heaters (such as cast iron radiators). Therefore base load supply and retrofitting are the rule. With the exception of Iceland, another prerequisite prevails respective to the geothermal resource to heat load adequacy. Both resource and demand need to be geographically matched.The two major components of a typical GDH grid are the geothermal loop and heating grid mains, interfaced by the geothermal heat exchanger. Modern doublet designs (in known areas) include two wells drilled in deviation from a single drilling pad.

Bottomhole spacings are designed to secure a minimum twenty year span before cooling of the production well occurs. Well depths (deviated) of 2000 to 3500 m are not uncommon; often located in sensitive, densely populated urban environments, they require heavy duty, silent rigs (up to 350 tons hook loads, diesel electric drive). Similar environmental constraints apply to periodical well maintenance (workover) operations which occasionally take place in landscaped sites. Fiberglas lined production/injection wells, first completed in 1995, are a material solution to steel casing corrosion. Continuous downhole chemical inhibition lines are another alternative to defeat corrosion/scaling shortcomings in hostile thermochemical environments.

Geothermal fluid production is usually sustained by artificial lift, i.e. submersible, variable speed drive, pump sets of either the electric or (enclosed) lineshaft type. Whenever self flowing production may be substituted, low well head pressures and subsequent escape of solution gases require the installation of a degassing/abatement unit. To combat corrosion damage and ease periodical cleaning, geothermal heat exchangers need to conform to titanium plate design and manufacturing.

Back up heat, below outdoor transition temperature (5 to 10 °C), can be supplied partly by heat pumps and totally by boilers. Heat pumps of the water/water type may upgrade geothermal heat recovery, from heat exchange alone, by depleting rejection temperatures and boosting grid distribution temperatures downstream from the geothermal heat exchanger. Accordingly, various heat pump configurations may be contemplated and heat pump units combined in either serial, parallel or hybrid modes. In several instances (Denmark, Germany, Iceland) absorption heat pumps, often associated with geothermal Combined Heat & Power plants (CHP), have been successfully implemented.

Geothermal district cooling is actually poorly developed in Europe, hardly 30 Mwt installed cold power. This development issue which could provide additional summer loads to GDH systems should therefore be challenged by geothermal operators (and users). Cooling based on absorption chillers (heat pumps), using water as a refrigerant and lithium bromide(or ammoniac) as an absorbent seems an appropriate answer, provided minimum geothermal temperatures stand above 70 °C. The refrigerant, liberated by heat from the solution produces a refrigerant effect in the evaporator when cooling water is circulated through the condenser and absorber.