Geothermal potential in Malta

It’s no secret that for environmental, economic and strategic reasons, the exploration of energy sources alternative to fossil fuels and hydrocarbons has been paramount in hopes of decoupling once and for all. So far, Malta has run a mildly successful campaign promoting domestic rooftop solar and paving the way for offshore wind but still far away from its 25% renewable target.

These two sources are obviously Malta’s best bet as they are the lowest hanging fruit with the least amount of uncertainty. However, there are two main features of thermal power generation that renewables have a hard time replicating: inertia and responsiveness.

To explain these terms briefly, inertia is the ability of the generator to remain stable and resist change upon a change in demand, and responsiveness is the ability to adjust output when faced with such demand-induced instability.

While some of these issues can be worked around by deploying energy storage and collaborating with the meteorological office to estimate power production, intermittent renewals will still need a fast-reacting source like LNG to ‘plug the holes’ whenever they come up against unforeseen consequences.

Geothermal energy is a renewable energy source that runs a steam turbine like any other thermal power plant.

The main difference is that instead of burning a fuel, the water is sent down a deep borehole where it can be heated to high temperature from the Earth’s core heat. The presence of a turbine in the system grants the power plant the same inertia and responsiveness present in traditional thermal power plants.

So, what’s the outlook for geothermal energy in Malta? It’s probably worth noting at this point that geothermal energy can be used for two main applications: heating/cooling or electricity generation. Heating and cooling have a very low barrier to entry as borehole depths can be anywhere from 25m deep to 200m deep, which has been used in Malta on a large scale already for the parliament building.

Using geothermal heat for electricity generation on the other hand has significantly steeper requirements. Since borehole heat increases with depth, it has to be deep enough to achieve at least a temperature of 150°C.

The higher the temperature beyond that, the higher the generator’s efficiency. The required depth is highly dependent on the land’s geology and its geothermal gradient.

Places like Iceland, Italy, Japan and Indonesia are lucky to have high temperatures available at surface level or at very shallow depths, however the average global geothermal gradient is a discouraging +30°C/km placing average required borehole depths on the order of 4km. This can cost between €1m-€10m per borehole for the bare minimum of power output.

In Malta we have no data about what type of geothermal gradient we can expect upon drilling because such surveys were never carried out.

The only boreholes of such magnitude ever drilled on land were the 3km deep, Naxxar-2 borehole drilled by British Petroleum while surveying for oil back in 1958; the second one was the Madonna taż-Żejt borehole that went all the way to 8km and cost €33m.

However, there are good reasons to suspect that Malta might offer favourable conditions for such a power source. Firstly, and most obviously, Malta is surrounded by volcanic land masses like Pantelleria, Linosa and Sicily implying that below the Maltese sedimentary rock there might be significant thermal reservoirs. Secondly, Malta happens to be on a stretched-out region of the Earth’s crust making it relatively thin thus allowing more heat to percolate through to shallower depths. However, we obviously won’t know unless we actually start drilling and measuring.

Malta is surrounded by volcanic land masses like Pantelleria, Linosa and Sicily- Jeremy Sacco

What are the costs, gains and opportunities here? By far the most expensive cost will be the boreholes. At a price tag that rises exponentially with depth, a base-load power plant would need at least two deep boreholes per megawatt of €1m-€10m each. However, test boreholes deep enough to allow an extrapolated estimate by geologists are very unlikely to exceed the €1m per hole, after which only the promising locations will be taken further to full depth.

Regarding funding, this could be a joint venture between science and energy budgets. A reasonable power plant to compare to would be the Blue Mountain Faulkner 1 in Nevada, the US.

The plant is a 50MW binary-cycle plant, with five boreholes at 2.4km depth (a temperature of 150°C). This project cost $180m from start to first operation. One could suppose that with the same cost Malta could be able to drill less boreholes to deeper depths as necessary to start, then in the future add more boreholes as deemed necessary.

Regardless of the geothermal outcomes it is still worth surveying the boreholes thoroughly both in terms of academic purposes and for hydrogen or hydrocarbon deposits that could still be exploited.

What if there’s no viability, would it be millions of euros down the drain? Not necessarily. While it’s incredibly unlikely to not break 100°C at 4km depth, the temperature will still most likely be over 80°C. A binary-cycle system can work with temperatures as low as 50°C but operating at such conditions would severely hinder efficiency.

Implying that for higher efficiency the temperature entering the turbine needs to be made as hot as possible.

This would leave two ways forward: either augment the heat coming out of the reservoir or augment the heat content in the reservoir itself.

To maintain the system as a renewable energy source, a viable augmentation for Malta is a hybrid thermal power plant that combines geothermal and solar-thermal heat.

Solar-thermal collectors could heat thermal oil to well over 400°C or boil water on a hot summer day. The heat from which could either be used to drive the turbine directly in high demand periods or pumped down the boreholes where the heat can be stored for later use. Another option for solar-thermal augmentation is to collect heat from underneath street tarmac which during summer can easily maintain a constant 60°C doubling as an urban heat mitigation strategy.

Augmenting the geothermal reservoir with solar-thermal heat also makes that heat energy accessible during night-time the same way that a battery would. Once again, the technology to augment medium temperature reservoirs could be developed domestically with appropriate research and energy funding.

The market for a geothermal system that cuts drilling costs in locations without meaningful geothermal reservoirs is a big one that’s constantly on the lookout for a breakthrough technology to get their hands on, thus potentially giving a return on investment.

Jeremy Sacco is a physicist, and an advocate for technological advancement in economic and energy sectors.