Technology

To be candid, it was love at first sight, or rather first understanding, realising the sheer elegance of the heat pump technology that takes heat from the surrounding environment – we call it “locally sourced” – and boosts that heat to the target or “sink” using electrically driven pumps or compressors.   We really like how a heat pump efficiently converts electrical work into both transferring the heat from the source to the sink, but also can increase the temperature of the heat at the same time.

Domestic heat pumps will be familiar to many people, but more so if you live in European country like Sweden; they are remarkably less common (so far) in other countries of similar climates such as Ireland and the UK.   Domestic heat pumps extract heat out of the air outside the home or from the ground under the building by evaporating a “refrigerant” in the pump (see the picture below).   That refrigerant is then squeezed in the pump with the compressor driven by electricity, from the grid, or from a local renewable source like solar PV.   The compression adds energy to the fluid which is then released in the “condenser”, transferred to warm spaces or make hot water.

Our heat pump sweethearts are very efficient.   The efficiency of the reversed Carnot Cycle is described as a Coefficient of Performance (“COP”).   The COP is the provided heat power divided by the work done by the pump.   For example, a domestic heat pump of 10 KiloWatts (KW) output with a COP of 2.5 uses an input electrical capacity of 4 KW and is drawing 6 KW from the ambient environment.   Domestic heat pumps, providing a few KW of heat to a home typically have COPs of 2 to 5, depending on both the pump itself and the amount of heat being drawn from the surrounding environment.  Ground-sourced (aka geothermal) heat pumps tend to be more efficient than air-sourced, but carry higher installation challenges and therefore costs.   However, heat pumps are inexpensive life partners in the long run, because very low running costs and efficient use of electricity to amplify “free” heat outweighs the installation costs.

Industrial heat pumps are similar to domestic ones, but much bigger.   Commercially proven machines can lift temperatures by as much as 80 ºC.  New technologies with the ability to provide up to 160 ºC and still maintain COPs greater than 2.5 are  being rapidly developed. They are typically deployed to recycle waste heat from a process that is powered by fossil fuel boiler combustion.   The recycling by heat pumps dramatically reduces the amount of energy wasted and, depending on the carbon intensity of the electricity, reduce the overall cost of the process heat.

Causeway Energies’s technological approach also uses industrial heat pumps of 100s of KW to MegaWatt (MW) scale.  However, the basic source of heat for the pump is not waste heat from fossil fuel boilers, but instead is deeper geothermal heat harvested in geothermal boreholes.  You can think of it as upscaled equivalent of the domestic similar to ground-sourced heat pump, but bigger – power ratings of MW rather than KW, and hotter – temperatures more than 120 ºC rather than less than 80 ºC.

The most attractive thing about both domestic and industrial heat pumps is the amazing heat decarbonisation that happens when the electricity used is largely from renewable energy sources (RES) such as wind and solar PV.   Using Irish or British grid electricity, or in some parts of the United States, where there is a high penetration of RES, allows for industrial heat to be delivered to customers from clean geothermal heat at 10% of the GHG emissions of a fossil fuel boiler equivalent.

Geothermal is a renewable energy source that has the potential to be low or zero GHG emissions, but can be “baseload” in character available 24/7/365 and dispatchable or accessible on demand.  Geothermal resources are also used for thermal energy storage, but injecting heat into the earth to be absorbed by the rocks, and then producing that heat when it is needed.

There are a number of ways that thermal energy held in rocks and/or fluids within the rocks in the subsurface can be harvested or reinjected.  The schematic cross-section illustrates a classification of these system archetypes.  It is based on an original diagram published by the British Geological Survey.  We have added several system types and provided additional detail on the source temperatures, distinguishing the use of heat pumps (Carnot cycle) for the provision of heat versus turbines (Rankine cycle) for the generation of electricity.

The classification assumes an average geothermal gradient of around 25 ºC/km and therefore is representative of most of the world’s continents away from areas of active extension or convergence of tectonic plates and volcanic hot spots, such as Ireland,  the UK and much of the United States.  Therefore “shallow”, less than around 500 m depth, also means “low temperature” or in the case of hydrothermal reservoirs, “low enthalpy”, with temperatures up to 25 ºC.

Use of geothermal energy in the world.

Shallow geothermal systems, depicted on the left-hand side of the diagram, are of two basic types and both most often involve the use of heat pumps to refine and boost the temperature of the thermal energy harvested in the geothermal boreholes.  A little over half of the world’s geothermal energy use is through shallow geothermal heat pumps.

There are two types of shallow system.  The most common is “closed loop” which is where heat is harvested to the borehole by conduction into a U tube or coaxial tubing in the borehole which contains a circulating fluid such as water or glycol-water mix.  There is no exchange of fluid between the rocks and borehole, the inefficiencies of conduction in rocks is allowed for through careful design of the number and length of borehole heat exchangers (BHEs) relative to the known heating (and cooling) loads through each year of the project.

“Open loop” is where there is an aquifer or groundwater resource in the shallow subsurface from which water can be produced, some heat extracted, and then most often re-injected into the rocks.  Although this system is much more efficient than closed-loop, requiring fewer boreholes, it is less common because the risks to the aquifer particularly if it is also a potable water source, need to be either avoided or carefully managed.

In our classification diagram, we have included mine water sources.  This system type is getting a good deal of attention in the UK for example, where old abandoned coal mines often naturally occur underneath or alongside centres of heat demand.  The water filled shafts and tunnels basically act as a big underground heat exchanger and that heat can be extracted either by open loop, or closed loop.

In deep geothermal systems, typically much deeper than 500 m, the heat is can be used for heating as with shallow systems or if above 80 ºC at the surface, for electricity generation.  All the heating systems we know of, except one, in the world are open loop and do not use heat pumps.  Most of the classic geothermal power generation plants sit in geologically hot areas where there are very hot waters or brines that flash as steam at the surface to drive conventional turbines.  But there is also a growing minority of binary turbines, where geothermal heat is used to heat another working fluid, with a lower vapour (or flash) temperature than water, making electricity in places like Nevada in the USA.

It is also worth pointing out clearly, that although the system depicted second from right to generate power is labeled “Enhanced Geothermal System”, in fact all current geothermal power plants use naturally occurring fluids contained withing naturally occurring permeability.  EGS is an exciting topic of research, development and demonstration, to create artificial permeability and even introduced water in otherwise hot, but dry rocks.  However, there are no commercial deployments on this technology as yet.

Similarly, we have depicted Deep Closed Loop Geothermal Heat Pump (GTHP) Array in the middle of the chart, but there is even less field pilot data on such deep closed loop borehole heat exchangers, let alone a commercial deployment.  Nevertheless, the technology is attracting attention and investment and Causeway is putting research effort into it too, because higher source temperatures in principle improve the efficiency of our heat pump systems.  Deep closed loop offers the same advantages as shallow closed loop, avoiding fluid exchange, contamination and induced seismicity, but is similarly hampered by the slowness of conduction in rocks.

The four verticals

We see four areas of innovation in our geothermal heat pump system.  We refer to these as “verticals”, each deserving – and getting – deeper investigation.  Our simple goal is to deliver higher temperatures to larger demands, everywhere, with economics that beat fossil fuels.

Rock smarts

On the right hand side, is the Subsurface simulation vertical.  This is about understanding the geology of the resource and being able to model and predict the thermal performance of the resource over decades.  This vertical is more about the capability of the team and our collaboration partners as it is about software or hardware.

Closed loop boreholes

Next to it, to the left, is the closed loop borehole heat exchanger.  This type of heat harvesting technology at depth is sometimes called Advanced Geothermal Systems (AGS).  While we will use open loop in many cases (if for example a good hydrothermal aquifer is present), closed loop is agnostic to rock type and the presence of fluids and permeability for the fluid to move through to the borehole.

The other technology that is being developed to deal with dry rock with no natural is permeability is called Enhanced (or Engineered) Geothermal Systems (EGS).  EGS uses hydraulic or thermal fracturing to artificially create a network of  permeable fissures for water to flow through, or  to enhance an existing natural network of fractures.  In principle EGS should be better than AGS, as fluid flow (advection, convection) is typically a lot faster in transferring heat than conduction through rocks.  However, aside from EGS also being as yet uneconomic in application, because it involves hydraulic fracturing (aka “fracking”) it has the additional challenge of being besmirched with the reputation that fracking has with opponents of the oil and gas industry.  If closed loop borehole heat exchangers can be optimized with respect to heat transfer and their drilling costs can be radically reduced then AGS in our view is the best candidate for deeper geothermal heat capture.

Drilling

Whatever type of heat exchange system is preferred, the cost of drilling the boreholes is a focus area.  Thirty to 50% or more of the upfront capital cost of any geothermal project is in the drilling of boreholes.  All geothermal startups in this exciting scene are interested in reduced drilling costs.  Some are even working as clean tech ventures to innovate new drilling technologies.  We are watching these firms carefully and we’re also looking for our own breakthrough ideas on reducing the expense of drilling boreholes.

Industrial Heat Pumps

Lastly, on the left hand side of the diagram is the heat pump itself.  There are a range of new products coming on to or at least towards the market that can handle sink (the process or target) temperatures of 165 ºC.  Furthermore the temperature lift from the source heat to the sink is increasing from 60 ºK in the past to over 100 ºK in the near future.  Causeway has its own research, development and demonstration program on industrial heat pumps and how to best integrate with the other three verticals to optimize the economics of the system.  We’re making good progress, having filed our first patent application on June 26th 2023 and hope to build a prototype of the patented system later this year.