Going green with geothermal: What every developer needs to know

by sadia_badhon | April 9, 2020 10:50 am

By Lane Theriault

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The way buildings are being constructed is changing quickly, largely out of a desire for a greener built environment. Municipal building codes are now becoming more stringent, resulting in greater energy efficiency. As a result, developers need to incorporate sustainable technologies to meet these demands.

Current building codes have an increased focus on energy efficiency. If a developer is looking to meet these metrics, there is no better place to start than with the heating and cooling plants within buildings. Aside from being one of the most energy-intensive parts of a building, it is where almost all of the onsite greenhouse gas (GHG) emissions are created (Figure 1). This is an area environmentally focused developers must pay attention to.

Geothermal system

To implement a geothermal energy system, it is important to first understand what exactly it is. The simplest way to describe geothermal energy systems is to imagine the ground as a large heat battery—with a one-year charge—that cycles this heat in and out of the earth. All geothermal systems rely on heat pump technology. The pump removes heat from the building and sends it into the ground. The heat travels through a series of fluid-circulating pipes in contact with the ground, transferring the heat from the fluid to the earth under the building. In the winter, the flow reverses, and the heat is extracted out of the ground to warm the building.

This process makes geothermal energy efficient. It functions without creating excess energy to heat and cool the building, and instead moves the existing energy within the building over the course of the year.

Setting up the plant

A geothermal heating and cooling plant can be set up in two ways:

• full; or
• hybrid.

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Hybrid systems incorporate conventional equipment (e.g. the boilers and cooling towers) to supplement the geothermal field while the full assembly is usually absent of these.

Hybrid systems can usually be more capital efficient providing more return on investment (ROI). This is due to different energy demands for buildings. Most HVAC plants are sized to meet the highest heating and cooling loads of the year, also known as the peak capacity. However, those peaks are very few and far between during most of the year and only a small fraction of that capacity is used.

In general, there is a dichotomy between geothermal and conventional technology. Geothermal boreholes are costly to drill, but once in place, are inexpensive to operate. Conventional HVAC equipment is the opposite, being less expensive to install, but costly to run, maintain, and input energy. Hybrid systems take advantage of this by essentially splitting the work of the plant into two pieces:

• the peaks; and
• the baseload.

In designing a system with geothermal handling or the baseload, one can justify the higher initial capital costs because of its high utilization and low operating cost.

Conversely, it is difficult to justify the high capital cost of meeting peak load capacity with a geothermal system when one could just install inexpensive conventional equipment that runs infrequently. In Canada, the optimal ratio of hybrid geothermal to conventional is about 30 to 70 per cent respectively, although other factors can affect those numbers. Figure 2, provided by Reshape Strategies, an advisory and development services provider for energy and infrastructure, illustrates results from a feasibility study on hybrid systems.

Advantages and disadvantages of a hybrid system

Aside from the capital efficiency feature, hybrid designs also have several other advantages over their full geothermal counterparts (Figure 3). An important factor is the ability to easily balance the energy loads over the year. The idea that the ground acts as a heat battery implies the same amount of energy coming in and out of the field is needed every year, or else the field could get too hot or too cold, worsening its overall efficiency. Hybrid systems have an embedded ability to prevent this from happening. If the seasonal balance is unusual, the conventional system can be run for a longer period of time to restore the equilibrium.

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Additionally, hybrid systems can act as a backup to the geothermal system, if needed, although the latter is more reliable than a conventional plant. For example, if the system is set up with the proportions described above, there will be a conventional assembly capable of handling at least 95 per cent of the season’s capacity and offer reassurance, if there is any worry about the geothermal system’s performance.

The use of hybrid equipment, particularly a boiler, can provide the ability to run high temperatures at certain times, whereas a full geothermal system can operate only at ‘ambient’ temperatures. The geothermal loop ambient temperatures can approach freezing levels in the winter, and that, in turn, means a big step up in temperatures required for effective heating. When this delta is large, heat pumps need to work harder and, therefore, need to be sized larger to deal with this ‘de-rating.’ This can mean heat pumps cost more and use more space in floor plans. The use of a boiler deals with de-rating by running the water loops at higher temperatures on the coldest days of the year, also known as design days, and help avoid oversized heat pumps.

Time constraints are another consideration for developers when installing a geothermal system, which can often lie on the critical path. In this case, a hybrid system, which inherently requires less drilling than a full one, will take less time to drill. This is an important factor when considering the ‘time value of money’ most developers are focused on. Further, because hybrid systems require fewer boreholes, they are often the only option if the site is small and cannot fit a field big enough for the entire building load.

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Going with a hybrid system does not entirely eliminate natural gas consumption onsite, leaving some exposure to natural gas that a full geothermal plant would eliminate. However, if 80 per cent of the energy is coming from the geothermal field, the system is 80 per cent future-proofed and is 20 per cent ‘unhedged’ against any rise in natural gas prices or other potential legislation prohibiting gas use.

The other disadvantage with a hybrid system is it still requires the mechanical space of a full conventional plant. This is where site-specific factors might cause people to reconsider the optimal geothermal-conventional mix. If the mechanical space can be repurposed into useful floor area, it may be more economical to build a larger geothermal field. In markets like Toronto and Vancouver, a few thousand square feet of mechanical space recovered can amount to some million dollars.

Going 100 per cent geothermal may not be needed to achieve the goal of maximizing a building’s sale value. For example, the geothermal field could be sized large enough to eliminate a cooling tower that might reduce a mechanical penthouse height to allow for an extra floor, while still being hybrid on the heating side.

Setting up the air side

Geothermal heating and cooling systems can work with any type of hydronic in-suite equipment, whether or not fan coils, heat pumps, or even variable refrigerant flow (VRF) are preferred.

Although fan coils are likely the least expensive in-suite equipment, they are often not a good choice for interfacing with a geothermal loop. Two-pipe fan coil systems cannot ‘load share,’ which is a convenient way to improve efficiency in the shoulder seasons by matching co-incident heating and cooling loads in the building. A fan coil configuration will also require large, centralized, water-to-water heat pumps that are expensive and take up mechanical space, usually sub-grade.

This usually makes fan coils a less attractive option to in-suite heat pumps, which can do the same work on a distributed basis and eliminate the need for centralized water-to-water heat pumps. The drawback with this setup is the noise associated with heat pumps, although this is less of a factor if the units are small (1.5 tonnes or less seems to be acceptable).

A VRF configuration works similar to in-suite heat pumps, but tends to be more efficient as no heat pump compressor energy is needed to load share. It also deals with the noise issue by having a single heat pump in a central mechanical closet on each floor that is away from the living spaces. Additionally, VRF has the potential to save on distribution piping costs needing only one set of risers versus one for every two units per floor that are more typical in a heat pump or fan coil design. On a tall tower, this can be a significant sum.

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Drilling considerations

The ground’s ability to conduct and store heat does not significantly vary from site to site, but the cost of drilling can be quite different. In most cases, the depth of rock is going to be the most important geological consideration when examining the economics of a geothermal field. For example, in the Greater Toronto Area (GTA), the shallowest part of the ground is typically made up of unconsolidated clays and sands, which is easy to drill through, but is prone to collapse. This part of the well needs to be cased, counterintuitively, making the shallowest part of the hole the most expensive.

Widely speaking, there are three distinct methods of drilling and installing a borehole field under a large building (Figure 4).

Bottom of pit

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After the excavation is complete, drilling rigs are brought in (or craned in) to drill holes at the bottom of the pit. Although this is the least expensive form of drilling, it is often the least desirable for developers. This method is likely on the critical path, as little foundation work can move forward before the drilling and piping connections are completed. If the delay persists, the cost can be expensive when considering the time value of the money spent to excavate, and the setback in completing the project.

Surface

This method addresses the critical path impact of drilling from the bottom of the pit. Boreholes are drilled prior to excavation, and pipes are plugged and cut just below the excavation depth. After cutting, excavation can proceed unimpeded, with only minor work needed to tie in the boreholes once the excavators reach the foundation depth.

This method can often avoid many critical path delays. Since there is no need for a permit to drill a geothermal borehole, if there is a vacant site awaiting a building permit, drilling can be completed without any impact to the construction schedule.

Surface drilling is more expensive than the bottom of pit method as installers must drill through ground that will eventually be excavated. That means paying for drilling that does not result in any heating and cooling capacity, increasing costs for comparable designs.

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Parking structure

Most recently, installers have been using electrically driven, low overhead drilling rigs to drill from the bottom of the parking structure in both new and existing buildings. For new buildings, excavation proceeds normally, and the piping to connect the boreholes is laid once the foundation level is reached. Boreholes are drilled after and can be delayed up until immediately prior to the plant being commissioned.

Although this is the most expensive form of drilling, the ability to delay capital spending can be worth it when considering the time value of money. Problems arise when there is a high water table or in a bathtub foundation design where the membrane cannot be perforated. In such cases, drilling from the surface can be a more effective choice.

Other opportunities

In certain cases, there may be other opportunities that can be taken advantage of when designing the system. In the same way, a heat pump system can share coincident heating and cooling loads between units, space conditioning and any other thermal loads in the building can be shared. This is especially true when it comes to a group of buildings that are cooling dominant, which is not uncommon in Canada. The heat created from these cooling loads can be used to warm domestic hot water, snowmelt systems, or even a pool. However, it is important to consider the timing of these loads. For instance, a snowmelt system is unlikely to be used in the summer when the cooling loads of the building are high.

Whatever way works best to integrate a geothermal heating cooling system into a building, it is sure to have an impact on its carbon footprint. Every project is unique and comes with certain challenges, but there is almost always an economical way to implement geothermal once all the possibilities available are explored.

[8]Lane Theriault is the founder of Subterra Renewables, a geothermal district energy developer. He was also employed on the M&A team at Citigroup specializing in large, complex private and public market transactions. Theriault is a chartered financial analyst and received his master of business administration from the University of Toronto. He can be reached at lane@subterrarenewables.com.

Source URL: https://www.constructioncanada.net/going-green-with-geothermal-what-every-developer-needs-to-know/

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