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  • Writer's pictureThe Heat Beat

What’s Heat Worth? Consider the Crown Joules.

By Bob Metcalfe.

Let’s drill for heat instead of oil. It seems a simple concept right? Indeed leveraging the expertise, technology, intellectual capital and even the workforce of the oil and gas industry to pioneer the future of ubiquitous, clean geothermal baseload is a foundational mission of GEO. But while the technological details of drilling deep, very hot and high pressure wells in the heat and fossil fuel mining contexts are largely the same, the concepts diverge significantly when attempting to quantify and compare the value of the resources.

Drilling for fossil fuels and heat have the same ultimate outcome – the production of energy. In the first case, liquid assets (oil and gas) are extracted from the ground, routed through the midstream/downstream and eventually to power plants, which burn them to produce steam. The steam turns turbines, which produce electricity for the grid. In the second case, mining for heat, we eliminate the need to burn a fuel to produce steam, and along with it the need to transfer, pipe, truck, store and deal with a fossil fuel asset, as well as the CO2 that is created by burning the fuel. In mining for heat, we simply engineer ‘radiators’ into deep hot bedrock, and circulate fluid in those wells which reach supercritical temperatures. We then run turbines directly with the emerging steam, producing energy for the grid.

So how can we compare the economics of these two systems when they do not share a common unit of measurement? Energy is measured in joules -- watt-seconds. And since the tippy top of a drilling rig is sometimes called its “crown,” let’s focus for a moment on what I’m calling…Crown Joules.

Units of measurement of interest to the oil and gas industry are terms like barrels (of oil) or barrels of oil equivalent. Obviously these units of measurement are not easily transferrable to the geothermal context, but let’s attempt a ‘barrel to barrel’ comparison of fossil fuels and heat.

Crude oil pumped to the surface is typically measured in units of barrels, a standard unit, each of which is 40 gallons or 0.16 cubic meters. How many Crown Joules are there in a barrel of crude oil? The barrel of oil equivalent (BOE) is approximately 6.12 gigajoules (GJ) of energy.

What if instead of crude oil we surfaced Crown Joules in boiling water? To heat up water, it takes 4.186 joule per gram per degree Celsius (J/gC). Let’s start with water at the average surface temperature of 14C (57.2F). One cubic meter of water weighs 1,000 kilograms. A barrel of water weighs 0.16 cubic meters times 1,000 kg or 160 kilograms. To heat a barrel of water from surface temperature (14C) to boiling is 4.186 X 160,000 X (100C-14C) or 57.6 megajoules (MJ).

So, comparing Crown Joules, a barrel of surfaced crude oil holds 6.12 GJ compared to a barrel of boiling water, which holds .0576 GJ. The Crown Joules in a barrel of crude oil are more than 106 times those in a barrel of boiling water.

This factor of 100 difference reflects the relative strength of the energy in the chemical bonds of hydrocarbons, released by burning (oxidation), versus in the kinetic energy of molecules (heat). If geothermal is to be economical feasible, we have to find ways of improving on that factor of 106. How can we achieve this? We need to increase the value of the steam we bring to the surface – and that is accomplished by increasing the temperature of what we are mining. Put simply, we need steam hot enough to make the economics attractive.

Now, let’s utilize GEO’s goal of steam mining at 350C. What if the Crown Joules of a barrel of water are brought to the surface as 350C steam?

Assuming our barrel of water started at 14C, it would need to absorb energy to get it to 100C water, then more energy getting to 100C steam, then more energy to 350C steam, as follows:

Ignoring temperature and pressure effects for now: The specific heat of water is 4.186 J/gC. Water’s heat of vaporization is 2,257 J/g. The specific heat of steam is 2.09 J/gC.

The energy required to heat a barrel of water to 350C steam from 14C water:

14C water to 100C water: 160kg X 4.180 J/g X (100C-14C) = 57,516,800 joules

100C water to 100C steam: 160kg X 2257 J/g = 361,120,000 joules

100C steam to 350C steam: 160kg X 2.09 J/gC (350C-100C) = 83,600,000 joules

Total energy to take a barrel of 14C water to 350C steam: 502,236,800 joules

So what’s heat worth? In the most basic terms, when utilized for energy production, the Crown Joules in ‘one barrel’ of 350C steam are worth 1/12th those in one barrel of crude oil. And this is where the founding mission of GEO takes center stage. Technological development and innovation has played a central role in cost reduction of new energy technologies, and it will do so in next generation geothermal energy concepts like closed loop systems.

Let’s consider solar and wind energy as an example. Lazard’s annual Levelized Cost of Energy analysis reported that over the past ten years, the cost of solar (PV) and wind energy energy production has dropped 88% and 69%, respectively. This is an extraordinary decline that is expected to continue with further investment and technological development. The National Renewable Energy Laboratory (NREL) has projected that advances in solar energy technologies could drive down costs an additional 80% by 2050, and an additional 60% cost reduction may be achieved during the same time period through industry growth and economies of scale.

Thus with robust industry engagement and investment, buoyed by enabling technological advancement, advanced closed loop geothermal baseload could quickly become competitive with fossil fuels. This is particularly true should government incentives and subsidies for geothermal R&D reach parity with those that have driven the solar and wind industries over the past decade.


Bob Metcalfe is Professor of Innovation and Murchison Fellow of Free Enterprise in the Cockrell School of Engineering at The University of Texas at Austin. He is Principal Investigator of the Geothermal Entrepreneurship Organization (GEO) in the Texas Innovation Center.

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