Two of the primary thermal heating technologies—electric resistance heating (ERH) and thermal conduction heating (TCH) – have many similarities, and it can be hard to figure out which one is best suited for a specific project site. But making the wrong call on technology can have expensive repercussions, which is why it’s critical to know the pros, cons, and differences of each.
In this blog post, I’ll outline the similarities and differences between ERH and TCH and explain which site conditions each is best suited for. I will also identify the kind of site data you need to ensure you’re utilizing the most cost-effective option.
How are ERH and TCH similar?
ERH and TCH are both very robust technologies that have been used successfully around the globe to clean up sites contaminated with organic chemicals.
Both technologies utilize electrical power to deliver energy to the subsurface to sufficiently raise the temperature to volatilize and mobilize contaminates to the extraction wells. In fact, despite different heating mechanisms (Joule heating for ERH and thermal conduction heating for TCH) they both use essentially the same amount of energy to heat and treat a given site.
This is because the energy required to treat a site is a function of:
- The volume, soil and water mass of the treatment zone
- The amount of groundwater inflow
- The amount of conductive and convective heat losses
- The COCs and their boiling points
- The remedial goals
The amount of energy required to treat a site is not a function of the technology or energy delivery mechanisms. One technology is not magically more efficient at heating the subsurface than the other. This applies to ERH and TCH. For Steam Enhanced Extraction (SEE), energy usage is higher because of the requirement for substantial liquid extract to maintain hydraulic control.
How do ERH and TCH differ?
Although they are very similar, there are many ways in which these technologies differ. For ERH, the primary heating mechanism is passing current between the electrodes distributed throughout and around the treatment zone, while for TCH, the mechanism is thermal conduction of energy and heat away from electrically powered heating elements located inside the heater wells. ERH is only able to heat up to the boiling point of water (100°C or slightly higher if below the water table). TCH can treat to temperatures up to 350°C because the heaters operate at 600 to 700°C, and energy moves out to the surrounding soil between the heaters by thermal conduction as a result of the significant thermal gradients.
Another difference between the two technologies is the installation of the required infrastructure. ERH electrodes are typically 4” to 8” in diameter and require a large borehole (10” to 12”), which is then backfilled with electrically conductive material (e.g., graphite or steel shot) to maximize electrical continuity with the soil. Meanwhile, TCH heater casings are typically 3.5” in diameter and can be installed in 6” diameter boreholes before being backfilled with thermally conductive, high temperature grout. For most sites, the spacing between ERH electrodes will range between 16 and 20 feet (18 ft typical), while for TCH the spacing between heater wells is typically 14-17 feet for 100°C applications (15 ft typical).
Interestingly, when considering the total cost of installation (well materials, drilling, investigation derived waste [IDW], and time/oversight costs), the cost to install an ERH wellfield with electrodes spaced at 18 feet at a moderate depth site (30 to 40 feet) in unconsolidated materials is very close to the cost to install a TCH wellfield in similar conditions with heaters spaced at 15 feet. Costs do vary more substantially between ERH and TCH when the site is deep (>60 feet) or extends into bedrock, with TCH wellfield installation costs being less expensive than ERH.
How do you choose the right technology?
When deciding or selecting which technology will work best and be the most straightforward to implement for your site, there are four major points to consider.
- What are the target contaminants being remediated and what are the cleanup goals? If the contaminants of concern (COCs) are high boiling point semi-volatile organic compounds (SVOCs) or polychlorinated biphenyls (PCBs), then TCH would need to be implemented to achieve the higher temperatures required to volatilize the heavier molecular weight compounds and meet the typical stringent remedial goals. If the contaminants of concern have a boiling or co-boiling point below 100°C, then either ERH or TCH will work.
- What is the soil resistivity at the site? Soil resistivity is extremely important when deciding if ERH or TCH is right for your site. It does not affect TCH heating or treatment efficiency, but it can have a huge impact on ERH performance. This is because soil resistivity or its inverse, electrical conductivity, dictates how readily the soil at a site will pass or conduct electricity and how much heating will occur for a given current flow. The sweet spot for optimal ERH performance is when soil resistivity is generally between 10 and 500 ohm-m.
A site with very low resistivity (high electrical conductivity) runs the risk of requiring large amounts of electrical current, beyond the capacity of typical electrical supply and distribution equipment and electrical cables, to get efficient heating. When the soil resistivity is too high, this creates a situation where high voltages (>>480V) are required to push sufficient current through the subsurface to get adequate heating. In this situation, special high voltage rated equipment must be used and significant efforts must be made to ensure the safety of near-by utilities, workers, building occupants, and equipment from stray currents.
If known before design, certain modifications can be made to the ERH approach to ensure successful performance at sites with high or low soil resistivity (i.e., tighter electrode spacing, increase water injection in the electrodes, etc.). However, for some sites, the resistivity is so high or so low, or a combination of the two scenarios, that it is very difficult to safely design and operate a system to achieve adequate and uniform heating. The good news is that in these situations, there are other technologies, such as TCH or steam enhanced extraction (SEE), that can more reliably provide good performance for lower costs.
- What is the depth to groundwater and the potential dryness of the vadose zone? Both ERH and TCH can work above and below the water table; however, ERH can run into potential issues if the vadose zone portion of the treatment zone is excessively dry. This is because the electrical conductivity of the soil is highly dependent on moisture content. Depending on the soil make-up (i.e., dry sands), ERH may not be able to inject enough water around the electrodes to achieve sufficient widespread wetting to ensure sufficient and uniform current flow through the ground for adequate heating. For example, if your site has a water table at ~100 feet bgs and the target treatment depth was ground surface to 60 feet bgs in a sandy/gravely soil, this would not be an ideal ERH site. Maybe not impossible, but not without considerable design efforts and performance risks. When you have the option of TCH, which will work very well in this type of setting without risks or challenges, then why force a technology on a site? Make your life easier and ensure your project’s a success by choosing the best technology fit for your specific site conditions and remedial objectives.
- What is the groundwater flux through the site? Since water requires 4x more energy to heat than soil, excess groundwater flux can be a significant challenge for both ERH and TCH. ERH may have the capacity to accommodate higher groundwater fluxes if the system is designed to handle the necessary current flows and has the flexibility to control and direct the current to the high flux zones (i.e., independently controlled stacked electrodes), but in general, if the site has a high ground-water flux (>1 ft./day), certain precautions need to be incorporated in the design for both ERH and TCH.
These design elements may include tighter spacing between the electrodes and heaters, incorporation of liquid extraction wells or sheet piling/slurry cut-off walls on the upgradient portion of the site, and allowance for more time and power to reach the target temperature and remedial goals. In some instances, steam injection can be combined with ERH/TCH to specifically target high flux zones, while ERH or TCH is used to heat the lower permeable zones.
Although in most cases ERH and TCH can be utilized interchangeably at a site, it is important to carefully consider how each technology operates and the specific subsurface conditions of a site to allow for the identification and selection of the best technology and approach for your project.
If you want to learn more about using ERH or TCH at an upcoming project site, watch our on demand webinar presented by our VP of Technology John LaChance, titled, ERH vs. TCH: How to Choose Your Thermal Heating Technology (And Why).
You can also contact us to chat about your specific site or to request an in-depth evaluation of the best technology and approach for your remedial objectives.
Kevin Crowder has 12 years of experience in working in the thermal remediation field. He specializes in the design, implementation and assessment of numerous ERH, TCH and SEE sites in the US, Canada, and internationally. As a Project Engineer, Kevin has been responsible for the design, oversight and implementation of over two dozen projects for…