3 ms·

The sales people all call it geothermal. We might need to retroname the old one... Geosteam? Borehole? Maybe a closed-cycle nitrogen or helium turbine would m

by ncmncm 4y ago

The sales people all call it geothermal. We might need to retroname the old one... Geosteam? Borehole? Maybe a closed-cycle nitrogen or helium turbine would make for fewer overhauls, thus lower maintenance cost, vs. exposing your turbines to superheated steam all the live-long day.

dredmorbius 4y ago
The two broadest classifications I see are Geothermal Power Generation and Geothermal Heating. These each have further subdivisions (and may be combined, as in a generation-heating facility, fairly common in Iceland). Generation can be divided in several ways. There are vapour and fluid-dominant systems, which refer to the dominant phase of the heat-transer fluid (water in all cases I'm aware), that is, liquid or steam. I'm not aware of any gas-based heat transfer, though you might want to add CO2 to your list as a potential working fluid. The phase-transition of steam from hot to cold side achieves much of the delivered power, and you'd be missing out on this with a purely gaseous medium. There are also both naturally occurring and enhanced sites. The former employ some existing geothermal site on Earth's surface, typically geysers or volcano. The latter can be sited anywhere, using a borehole. The difference being that drilling is far more expensive than tapping into an existing near-surface structure. The system Homer-Dixon is describing is a very deep borehole --- tens of kilometers rather than a few thousand meters. Geothermal heating utilises the direct thermal potential of the ground, rather than generating electricity (or as an adjunct to having generated electricity). These concepts range from existing / opportunistic siting such as hot springs or hot baths, to technically-coupled heat transfer and/or storage through heat pumps with a ground-loop component. There are also passive designs which simply utilise the insulating capabilities of soil or bedrock. Thick-walled adobe or stone structures thermoregulate by sheer thermal mass (possibly with other elements to increase or reduce solar gain and airflow). Sub-grade, semi-underground, or backfilled structures operate similarly. Seasonal thermal energy storage may be incorporated into single structures or at a community level in which excess summer heat is transferred to a thermally-stable geographic strata, and recovered from the same in winter. This requires that the subterranian layer not have a substantial heat flow, e.g., groundwater which would convey stored heat or cold away. There are also structures which exhibit an unwanted thermal effect over time. Many deep mines are hot and require active cooling of airflows. The London Underground subway system runs through rock strata which have absorbed heat from equipment and passengers for over a century, and whilst initially cool, is now unpleasantly warm. Plans to actively cool not only the void space (air) but the surrounding rock are now being explored.
- ncmncm 4y ago
  At issue is what we are expected to call most of these things. Home heating and utility-scale electrical power generation are different in kind, not just scale. They merit different language.
  - dredmorbius 4y ago
    Before the specific language can be agreed on, having a sense of what the distinct phenomena are. I do agree that present usage is often vague and similar terms are used to describe wildly different applications. "Residential" or "end-use" vs. "regional" or "grid scale" are among the differentiators I'm familiar with. Marketing is another driver of much confusion, as selling a ground-loop heat pump sounds much more impressive when described as "geothermal heat pump". The latter is ... stretching definitions at the least, if not openly misleading.

by ncmncm 4y ago

dredmorbius 4y ago

The two broadest classifications I see are Geothermal Power Generation and Geothermal Heating. These each have further subdivisions (and may be combined, as in a generation-heating facility, fairly common in Iceland). Generation can be divided in several ways. There are vapour and fluid-dominant systems, which refer to the dominant phase of the heat-transer fluid (water in all cases I'm aware), that is, liquid or steam. I'm not aware of any gas-based heat transfer, though you might want to add CO2 to your list as a potential working fluid. The phase-transition of steam from hot to cold side achieves much of the delivered power, and you'd be missing out on this with a purely gaseous medium. There are also both naturally occurring and enhanced sites. The former employ some existing geothermal site on Earth's surface, typically geysers or volcano. The latter can be sited anywhere, using a borehole. The difference being that drilling is far more expensive than tapping into an existing near-surface structure. The system Homer-Dixon is describing is a very deep borehole --- tens of kilometers rather than a few thousand meters. Geothermal heating utilises the direct thermal potential of the ground, rather than generating electricity (or as an adjunct to having generated electricity). These concepts range from existing / opportunistic siting such as hot springs or hot baths, to technically-coupled heat transfer and/or storage through heat pumps with a ground-loop component. There are also passive designs which simply utilise the insulating capabilities of soil or bedrock. Thick-walled adobe or stone structures thermoregulate by sheer thermal mass (possibly with other elements to increase or reduce solar gain and airflow). Sub-grade, semi-underground, or backfilled structures operate similarly. Seasonal thermal energy storage may be incorporated into single structures or at a community level in which excess summer heat is transferred to a thermally-stable geographic strata, and recovered from the same in winter. This requires that the subterranian layer not have a substantial heat flow, e.g., groundwater which would convey stored heat or cold away. There are also structures which exhibit an unwanted thermal effect over time. Many deep mines are hot and require active cooling of airflows. The London Underground subway system runs through rock strata which have absorbed heat from equipment and passengers for over a century, and whilst initially cool, is now unpleasantly warm. Plans to actively cool not only the void space (air) but the surrounding rock are now being explored.

ncmncm 4y ago
At issue is what we are expected to call most of these things. Home heating and utility-scale electrical power generation are different in kind, not just scale. They merit different language.
- dredmorbius 4y ago
  Before the specific language can be agreed on, having a sense of what the distinct phenomena are. I do agree that present usage is often vague and similar terms are used to describe wildly different applications. "Residential" or "end-use" vs. "regional" or "grid scale" are among the differentiators I'm familiar with. Marketing is another driver of much confusion, as selling a ground-loop heat pump sounds much more impressive when described as "geothermal heat pump". The latter is ... stretching definitions at the least, if not openly misleading.