Designing District Geothermal Systems: A Quick Guide
More and more now, we’re seeing our clients pursue large, ‘master planned’ community developments. These developments are fantastic opportunities to use district energy to meet the communities heating and cooling needs in an efficient and sustainable manner. Where traditional district energy systems have lacked appeal in less dense neighbourhoods, new ambient temperature systems that incorporate technologies like geothermal have great promise in developments that span large areas.
Benefit of Ambient Temperature District Systems
In the past, district energy systems were mostly ‘high temperature’ systems that were predominantly used for heating, not cooling. These systems have a number of disadvantages over a modern ambient temperature system. For starters, high temperature systems rely on moving water or steam that’s well over 100 degrees Celsius in the winter through ground that might be only 10 degrees Celsius. When the difference in temperature between the pipe and the surrounding area is high, energy losses along that pipe will also be high. The way that most of these systems have dealt with this issue is to use insulate piping, which is not only expensive, but doesn’t completely eliminate the problem. Additionally, these pipes can only be used for heating, and if cooling is desired, a second set of (insulated) pipes is needed. And if the heating and cooling are driven by traditional sources, the ability to share loads is non-existent.
By contrast, geothermal district energy systems don’t have many of these issues. Firstly, geothermal systems operate at temperatures similar to the ground temperature in both heating and cooling modes. This means that line losses are very minimal, so piping costs are lower because insulation requirements are reduced or eliminated, and the system can reach far further in an efficient manner than a high temperature system. Ambient loops can also provide simultaneous heating and cooling, and can do so on the same, single set of pipes. When this is happening, there’s an added benefit of not having to produce this energy at the plant, as the building in cooling mode would be rejecting the heat needed for the building in heating mode. This can be particularly useful if you have, say, a skating rink that needs continuous cooling and produces heat as a by-product which can be used elsewhere in the development.
Distributed plant vs. Centralized plant
Generally speaking, in low temperature systems, centralization reduces costs of district energy systems. Since line losses are low, there’s little benefit to any user being closer to the energy generation point, yet there’s a cost to adding additional connection points and pumps that are required for each individual segment. For every geothermal field you have, you need a new set of pumps and valves to connect to the central distribution loop, and a place to house them, which you may not have available in say, a single-family neighbourhood development.
If space is a concern, however, a distributed plant design may be more well-suited if it allows you to spread boreholes over the development. This can also be particularly useful in ‘modularizing’ the construction of the plant. Instead of building all the infrastructure on day one and waiting for users to connect to the system, plant capacity can be added when needed, lessening the financial ‘time value of money’ impact of building everything upfront. It is worth noting, however, that modularization can be achieved without distributing the plant across multiple locations – you can add boreholes right beside the existing field as you expand capacity.
The piping design is also an important consideration in optimizing the capital efficiency of the system. The more load you need on a given loop, the more flow you need in the pipe feeding that loop. If you need more flow, the pipes and pumps need to be larger. Additionally, the longer the distribution piping distance, the more pumping you need as well.
If we were to connect an entire neighbourhood on a single line, this line would have to be quite large (10+ inches), and the pumping power immense. Instead, we’ve found the primary / secondary loop configuration works best to address this tension. This means we would have one large pipe loop running through the centre of the development, with a number of secondary loops connected to the primary loop (see diagram below). In this design, we needed only 4’ pipe for the secondary loops, which cut the cost per foot dramatically.
Additionally, there are resiliency benefits to setting up the distribution system this way. If the whole system was on one loop, any problem with that loop would cause the entire system to go down. In the primary / secondary setup, problems on secondary loops can be isolated, minimizing the impact on the whole development, and limiting whole neighbourhood shutdowns to problem on the primary loop.
Any geothermal-based system will need to be ‘balanced’ in terms of heating and cooling loads over the course of the year. Geothermal systems are really just a heat battery, and so any major imbalance in loads will generally not be sustainable. With that said, some type of ‘hybridization’ is likely required to keep the system operating efficiently. If space for a larger central plant is available, adding conventional components here is likely the best solution when taking into consideration capital and operating costs.
On the flip side, many heat pumps are built with gas furnace or electric resistance backup in them, which can be used to balance the heating side at the final point of consumption. The economics on these solutions aren’t great, however – The electric backup is very expensive and inefficient to run, although incrementally not very expensive to install. The gas backup, though cheaper to run, requires a gas line to each home, which eats into the developer’s capital costs, and can also eat into the user’s operating costs, as most gas companies charge significant connection fees each month before any fuel is consumed.
By following some of these key design principals, district energy systems in most communities are economical for developer and resident alike. In our experience, we have been able to design systems in conjunction with our clients that not only help them meet their sustainability goals, but do so without increasing capital costs on the developer, and keeping consumers’ energy costs in line with conventional heating and cooling. With today’s technology, any developer building a large-scale development would be wise to consider a low-temperature, highly sustainable geothermal district energy system for their next project.