A geothermal plant is a heat engine like any other: it can never convert more of its extracted heat to electricity than the Carnot limit allows, set by the reservoir temperature and the ambient temperature — and because that gap is modest, the ceiling is far lower than a high-temperature steam plant's, often near 13% once binary-cycle losses are counted, so close to 85% of the extracted heat leaves as waste heat. Set the reservoir, ambient and reinjection temperatures, the brine flow, the reinjection fraction, and the reservoir size, then watch net electricity track the gap between gross output and the plant's own parasitic pumping load — and watch the reservoir itself over decades, cooling if you mine heat faster than it recharges unless reinjection and a sustainable extraction rate keep it steady. It all runs on your device.
Heat mined from the reservoir in, net electricity out — and where the rest goes.
Net power—
Gross power—
Carnot ceiling—
Thermal power—
Parasitic load—
Waste heat—
Reservoir temp—
Sustainability—
Capacity factor—
Power output
Energy split — electricity vs waste heat
Electricity & heat extracted
Reservoir temperature & power over the decades
Plant & reservoir schematic
About 85% of a geothermal plant's extracted heat becomes waste, not electricity. A geothermal plant is a heat engine: its ceiling on turning extracted heat into electricity is the Carnot limit — one minus the ambient temperature over the reservoir temperature, both in kelvin — and because a reservoir runs far cooler than a boiler or reactor, that ceiling is already low; real binary-cycle losses push the achievable conversion down further still, near 13% overall for a typical low-temperature resource. That leaves roughly 85% of the extracted heat to leave as waste heat. Efficiency isn't the only number at work either: the plant spends a share of its gross output as a parasitic pumping load circulating brine down injection wells and back up production wells, so net electricity is always lower than gross — and if you extract heat faster than the reservoir recharges, its drawdown over years erodes both the ΔT and the output, which is why reinjection and staying within a sustainable rate matter.
Reading the simulation
What the plant schematic, the energy-split bar and the reservoir plot are telling you — then what decades of extraction do to the resource.
1
The plant & reservoir schematic
Brine at the reservoir temperature drives the binary-cycle turbine and generator before the cooled brine heads back down through reinjection wells. The gauge beside the scene fills toward installed capacity as the net electrical output tracks the modest Carnot ceiling the reservoir-to-ambient gap sets.
2
The energy-split bar and the ceiling
The energy-split bar breaks the extracted thermal power down into three slices: waste heat rejected by the Carnot limit and the binary cycle's own losses, the parasitic pumping load that never leaves the plant, and what's left as net electricity — the smallest slice, and the only one that reaches the grid.
3
Reservoir drawdown over decades
Run the sim across up to 30 simulated years and the reservoir plot tracks reservoir temperature as extraction, natural recharge and reinjection compete. The sustainability readout reports whether the current extraction rate holds the reservoir steady or is drawing it down, while net electricity and heat extracted keep accumulating from the energy already produced.
How It Works
A hard ceiling on conversion, a heat balance that sets live output, and a reservoir drawdown that plays out over decades, not hours.
Hot brine rises through the production well, gives up its heat to the binary-cycle plant, and the cooled brine is pumped back down the injection well. Only a small share of the heat becomes electricity for the grid; pull heat up faster than the hot-rock reservoir recharges and it cools over the years.
1
The Carnot limit sets a hard ceiling
Any heat engine's efficiency is capped by the Carnot limit: one minus the ambient temperature over the reservoir temperature, both measured in kelvin. Because that gap is modest for geothermal resources, the ceiling itself is already far below a high-temperature steam plant's, and the binary-cycle plant's own second-law losses bring the achievable overall conversion down near 13% for a typical low-temperature resource — a limit set by thermodynamics, not by how well the plant is built.
2
Flow and temperatures set live output
Thermal power extracted is brine flow · specific heat · (reservoir temp − reinjection temp), and gross electrical power is that thermal power times the Carnot ceiling and the binary-cycle utilization factor together. Net electrical power is gross minus the parasitic pumping load — the pumps circulating brine claim a fixed share of gross before it ever reaches the grid. Raising the reservoir temperature or the reservoir-to-reinjection gap, or lowering ambient temperature, widens the ceiling and lifts output.
3
Drawdown, recharge and reinjection
Extracting heat faster than natural conduction recharges the reservoir cools the rock and fluid over years, a decline called drawdown that shrinks both the ΔT and future output. Reinjecting the cooled brine returns mass and residual heat to the reservoir, and staying within a sustainable extraction rate — at or below the natural recharge rate — keeps output steady for decades instead of declining.
Classroom activity
A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
High school, College10–20 minInteractive simulation
Learning objectives
Explain how geothermal energy is extracted.
Relate output to the underground temperature gradient.
Describe heat transfer from rock to fluid.
Variables to change
Well depth
Rock temperature gradient
Flow rate
Procedure
Set a depth and note the temperature of the extracted fluid.
Increase the depth and observe the temperature rise.
Change the flow rate and observe the effect on extracted power.
Observations
Record extracted temperature and power versus depth and flow rate.
Questions
Where does geothermal energy come from?
How does temperature change with depth?
Why does deeper drilling generally help?
How is the heat carried to the surface?
Is geothermal energy renewable?
Explanation
Geothermal energy taps the Earth’s internal heat, which increases with depth along the geothermal gradient. A working fluid heated by hot rock carries energy to the surface; drawing heat sustainably keeps it renewable.
Answer key (for instructors)
1. Heat stored in the Earth’s interior, conducted up through the crust.
2. It rises with depth (the geothermal gradient, ~25–30 °C per km on average).
3. Deeper rock is hotter, so the extracted fluid carries more usable heat.
4. A working fluid (water/steam) is heated by contact with hot rock and pumped up.
5. Effectively yes, if heat is extracted no faster than it is replenished from below.
Educators: link or embed this simulation freely in your LMS or course guide.
What is the Carnot limit on a geothermal plant's efficiency?
The Carnot limit is the theoretical ceiling on how much of a heat engine's thermal energy can become work, set purely by its hot and cold operating temperatures in kelvin: one minus the ambient temperature over the reservoir temperature. Because a geothermal reservoir runs far cooler than a boiler or reactor, that gap is modest and the Carnot ceiling is already low, and the binary-cycle plant's own second-law losses push the real electrical efficiency down further still — often near 13% overall for a typical low-temperature resource. No amount of engineering can beat the temperature-set ceiling itself — only narrow the gap between real efficiency and it.
Why does a geothermal plant reject about 85% of its heat as waste heat?
A geothermal plant is a heat engine, and every heat engine's electrical efficiency is capped by the Carnot limit — the fraction it can't convert to work is forbidden by the second law of thermodynamics itself, not by poor design. Because the reservoir-to-ambient temperature gap is modest, that Carnot ceiling is already low, and the binary-cycle heat exchanger, turbine and generator add their own losses on top; only around 10-15% of the extracted thermal power typically reaches the grid as electricity, with the rest leaving as low-grade waste heat. That rejected heat isn't always thrown away, though — many geothermal sites route it to direct-use applications like district heating, greenhouses or aquaculture, capturing value from the same low-grade heat a pure power plant would vent to the air or cooling water.
What's the difference between gross and net geothermal power?
Gross power is what the turbine and generator produce from the heat extracted out of the brine. Net power is what actually reaches the grid after subtracting the plant's own parasitic load — chiefly the pumps that push brine down injection wells and lift it back up production wells against reservoir pressure. That parasitic pumping load can claim a real double-digit share of gross output, so net electrical power is always lower than gross.
What is reservoir drawdown and why does reinjection matter?
A geothermal reservoir isn't an infinite heat source: pull heat out faster than natural conduction recharges it, and the rock and fluid cool over years to decades, a decline called drawdown that shrinks the reservoir-to-ambient ΔT and the achievable output along with it. Reinjecting the used, cooled brine back into the reservoir returns both mass and residual heat, and keeping the net extraction rate within the reservoir's natural recharge rate is what keeps output sustainable indefinitely instead of declining. Reinjection also helps maintain reservoir pressure, reducing the risk of land subsidence above the field, which is why nearly all modern binary-cycle plants reinject essentially all of the brine they extract rather than disposing of it at the surface.
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