Decarbonising cities: Retrofitting large fossil fuel heating systems with geothermal heat pumps

Figure 1: A schematic illustration of Causeway’s geothermal heat pump

The retrofit challenge

Causeway Energies has been in the engaging with our target market for a little over a year.  One of our key sectors is places – the built environment.  Here we are replacing gas and oil boiler heating with geothermal heat pumps. In Ireland and the UK, where the majority of our engagements are running, we find repeatedly in brownfield or retrofit situations the heating system is often designed for higher – 80 or 90 ºC temperature flow.  This is because the system has been designed around combustion-sourced heat, even though the space and water heating is to much lower temperatures.

Should the owner of the facility not want to change out the existing hydronic system this means that the replacement heat pump will need to deliver 80 or 90 ºC heat to the system’s heat exchanger.  For conventional industrial heat pumps, providing for heat demands 0.25 MWth and bigger in building like city hospitals, multistorey offices or residential complexes and heat networks, this can be a techno-economic challenge.  Let’s explain why.

Figure 2: An approximation of the relationship of Coefficient of Performance (COP) of a conventional heat pump and the temperature of the source heat in a scenario where 80 ºC is the required sink/flow temperature.

Geothermal heat pump economics

When compared to fossil fuel equivalents the competitive economics of heat pumps rely on 3 factors.

Firstly, because the cost of grid electricity per unit is typically three or more times expensive than natural gas it is important that the Coefficient of Performance (COP) of the heat pump is greater than three.  This means that the energy costs of running the system in operation compare favourably the gas predecessor.  Furthermore, it’s helpful for the COP to be four or more so that there is financial wiggle room to cover the amortization of the capital costs of installation.

This leads to the second factor in the techno-economic competition which is the capital efficiency of the heat harvesting system.  In essence we need as lower capital cost per Watt of thermal energy source as possible. For air source heat pumps, this is controlled by the size and therefore capital and circulation costs of Air Handling Units.  For geothermal heat pumps, it is the capital costs of the boreholes, whether open or closed loop, and parasitic energy costs of pumping and circulating the fluids in the boreholes.

The third factor is the COP itself which as described above acts as a fulcrum in the balance of capital costs and operating costs in the total levelized cost of the heat delivered.  There are several factors that control COP but a major one for conventional vapour compression heat pumps is the temperature lift, or the temperature difference between the source heat and the sink or flow heat.  Figure 2 illustrates this phenomenon for a range of source temperatures to deliver 80 ºC sink or flow.  The trend is not linear, but the positive impact on COP by higher source temperatures illustrates the control on COP exerted by the temperature lift.

“Deep” geothermal heat pumps

So, in the retrofit situations mentioned above, it makes sense to drill deeper for high temperature geothermal heat.  That’s part of Causeway’s proprietary approach to meeting this retrofit challenge.  Our patent-pending method and system uses boreholes down to 1 km depth, as unit drilling costs are relatively cheap to this depth.  While we take advantage of hydrothermal aquifers if present with an open loop design, we also have a novel closed loop system that doesn’t require pumping for circulation of the fluid that gathers the heat within the borehole.  Also, for closed loop systems, deeper boreholes mean more contact area with the heat-bearing rocks and therefore more Watts of thermal energy per borehole.  This helps answer another critical challenge in the urban environment – lack of space for the surface components of the borehole array.

There are further opportunities to optimize the geothermal heat pump solution for 80 ºC or more flow temperatures.  There are new heat pumps coming to the market now that can lift the temperature from source to sink by 100 ºC (or more correctly K) or more.  One key focus area in this development is the use of refrigerants that have properties to enable high lifts and yet be still have acceptable COPs.

For example, one novel heat pump cycle that Causeway is focused on is supercritical carbon dioxide (sCO2).  This fluid, at relatively low pressures and temperatures, behaves like a gas to expand or reduce in space, but is dense like a liquid and has superior heat absorption and release properties in the heat pump cycle.  In partnership with the Southwest Research Institute (SwRI) we have designed a heat pump cycle that raises the temperature by 100 K or more.  We have included two novel components to the heat pump engine to maximise its thermal efficiency and therefore COP.  This patent-pending heat pump system can take 25 ºC source heat and upgrade to as much as 150 ºC.

The combination of cheap geothermal heat and low operating costs of the heat pump is a breakthrough in renewable heat and is ideal for the retrofit challenge described in this article.  The levelized costs of the geothermal heat pump come out as similar to, if not better than, the gas boiler predecessor.  Our proprietary approach identifies the sweet spot between geothermal energy that is deep enough to maximize the source temperature (and COP) and not too deep that the escalation drilling costs offset the benefits of higher COP and lower operating costs.

While these technologies will add further efficiency to our projects, there are already many opportunities right now to replace fossil systems already with geothermal heat pumps with competitive economics, even with not including pricing of the emissions avoided.

If you have such a challenge and wonder if a deep geothermal heat pump will work for you, please do get in touch.