Power, energy, electricity, heat and geothermal – the importance of the right terminology

Power, energy, electricity, heat and geothermal – the importance of the right terminology Geothermal energy overview (source: GeoVision report by the U.S. DOE)
Alexander Richter 6 Mar 2020

Describing geothermal energy and what it has to offer, it is important to differentiate clearly between power and heat. Let's make sure we are using the right terminology to also help others understand the fantastic opportunities provided by geothermal energy to the global energy transition.

Let’s talk about geothermal energy … a while back I wrote an article on geothermal energy and how we can categorize its position within the renewable energy space of what it has to offer to the global energy transition. You can find the article here, and the overview below.

The categorization is important, as it looks at the potential role of technologies and what they have to offer to the energy transition. As we are facing climate change and the dire need to cut back in carbon emissions, each of the renewable energy technologies has a role to play. Geothermal energy in this context particularly shines in what it has to offer to cut back carbon emissions in the heating and cooling sector.

So in this context, we as an industry face a challenge, as we have so far not been able to clearly carve out the message of what we have to offer, or at least it is not arriving with general media or often not even in announcements by companies in the geothermal sector.

Part of the problem is clearly also a “lost in translation” element, I see far too often e.g. in Google Translate translations. So what am I talking about? I am talking about the constant mix of the terms “power”, “power plant”, “heating”, “power plants providing heating” etc.  While I don’t want to pick on a particular media outlet, a recent article by RECHARGE, so perfectly makes the case to point to the mistake we and others constantly make. The article refers to the fantastic story of the energy utility E.ON that plans to drill for geothermal energy in Malmö/ Sweden to fuel heating networks in the city, we reported.  The article correctly talks about the planned drilling for heat and the direct use of that heat to fuel a district heating network, yet the title simply is wrong. E.ON is not “to build deep geothermal power plants to heat Swedish homes.”, it is drilling for heat to be used in a “heating plant” to “heat Swedish homes.”


RECHARGE news on geothermal project by E.ON in Sweden, March 2020 (source: screenshot)

So what is wrong with that you might ask. The definition of the different terms of power vs electricity, energy generated, capacity installed etc can be confusing, so let’s briefly tap into this.

A good article of CleanTechnica from 2015 actually explains the difference between “power” and “energy” quite well, highlighting the fact that there is a difference of how much “power can be delivered at any moment”, this would be the kW or MW or GW, while energy refers to energy generated over time so in this case power over time, this would be kWh or MWh. So let’s take an example. When we refer to a 10 MW geothermal plant, then we talk about a capacity to deliver power at a certain time. So how much energy is then generated depends on the capacity times the hours it operates. A year has around 8,760 hours, so a 10 MW plant would deliver 87,600 MWh per year if it operates without any interruption.

So would this be the same for a plant providing heat and power? Essentially yes, yet it generates a different form of energy.

Here a few definitions:

  • Electricity, likely the easier form of energy we all know to charge our phones, computers, yet also “power” electric heating, here generating heat through an electrical appliance. – essentially defined as “effect of an electric charge, that powers e.g. a light bulb”.
  • Power, in the context of a “power plant” or “power station” is an “industrial facility for the generation of electric power”, and “power” in this context is the “rate of producing, transferring, or using energy, most commonly associated with electricity
  • Heat can either be generated through electricity, burning coal, wood or similar, as heat generated in industrial processes (often as part of electricity used) which could then be used indirectly to also generate electricity (think “heat power”), or be derived directly from geothermal energy resources.
  • Energy is likely the more wider and more complicated term. Energy can be contained in molecules (chemical energy), can be energy from electrical fields (electrical energy), gravitational energy, mechanical energy, and thermal energy (think heat), or generally energy being transferred in a given process of applied force.

So to summarize, energy describes in the sense of physics, a property that must be transferred to an option in order to perform work on or to heat the object. So for our purposes energy describes the capacity to generate electricity and heat (or even cooling).

In the context of how we describe geothermal energy and what it has to offer, it is absolutely crucial that we describe things correctly.

A geothermal “power plant” (or “station” used eg in New Zealand) generates electricity from heat. So drilling for geothermal resources tapping steam that then turns a turbine to generate electricity in a simplified description. In the context of heat, we are tapping geothermal resources, either directly heat from water resources (a reservoir), or tapping heat contained in rock and deriving it through a heat exchange system. We hence talk mostly about the “direct use” of geothermal resources in its purest form, namely “heat”.

So when a company like E.ON in the case mentioned above, is drilling for geothermal energy to tap the heat for heating purposes, it taps into geothermal energy directly. It will not build a power plant that would take heat to generate electricity and then provide heating. So the plant they will build will be a “heating plant” that derives heat from wells and – likely – put it through a heat exchange system in the plant that heats the water/ fluid used to operate the district heating network pipes to heat homes.

So the energy contained in geothermal provides the opportunity of it being used directly as heat, heat used to generate electricity (which in turn could be used to generate heat and cooling as well), and cooling through absorption chilling (here a good description).

This makes geothermal energy so attractive, as it provides the most direct and efficient source of energy for heating purposes. It directly uses heat for the purposes that need heat, e.g. heating, hot water, food dehydration etc. rather than indirectly through generating electricity first and then generate heat.

So back to the story of RECHARGE, let’s make sure we are using terms correctly and talk about plants in the context of what they do. “Power Plants” generating electricity and “heating plants” providing heat. Only if we use this correctly others will as well.

That is why you often find the specific description of what generation capacity a plant is providing, e.g. MWe for MW electric, or MWth for MW thermal … so a one (1) MWe plant would generate one (1) MWh of electricity per hour, while a one (1) MWth plant would generate one (1) MWh of thermal energy per hour – naturally under the assumption it operates with no disruption. This would then be the discussion on capacity factor or uptime defining how much energy is generated over a given time, so if a plant would only operate one day out of two, it would only generate 50% of the energy (electricity or heat) it could generate with the capacity installed. In our example of the 10 MW geothermal “power” plant, if it were to operate only 50% of the time, it would only operate 4,380 hours per year and therebey only generate 43,800 MWh of electricity per year.

Check out the fantastic glossary tool of the U.S. Energy Information Administration.

Note: the text was adapted slightly to correct a few linguistic elements in the earlier version