Geothermal Series – Part 1

Introduction

We are starting a new series! In this series, we are going to go through the design and build of our home-made geothermal system. This will be a multi-part series that will cover different aspects of the geothermal system, and how we came to the solutions that we did. As of writing this post, we are in the final stages of the project. Hopefully it will all come together!

Design Goals

The goal of going with geothermal is efficiency. Plain and simple. The building receiving this system is off grid, with solar panels and battery storage to run the electrical system. Heat and air constitutes a large chunk of energy demand on that system, with heat being the most demanding.

The original idea was to have gas heat as that would alleviate that draw on the system. However, there are no mini-splits that have a gas heat option, and a central heat system is just not practical in this case. It should also be noted that there are no mini-splits with a ground-source heat pump. Since we are committed to electric heat, we are determined to make it as efficient as possible to minimize effects on the battery.

A secondary goal to the project is to have a robust data collection system. The goal is to study the effect of a geothermal system on the ground temperature. We will combine multiple ground temperature and water temperature sensors, as well as an above ground weather station, to get the best overall picture of the system.

Problems

Where do I begin with the problems? Like I said earlier, there are no ground-source heat pumps (GSHP) available on mini-spit sized units. Most consumer grade GSHP’s are three tons or larger. The building we are putting this in is less than 1000 square feet, so something that large is quite unnecessary. Another problem with consumer grade GSHP’s is their costs. We are talking around ten thousands of dollars for the heat pump and then even more to install the associated ground loops. It is not uncommon here in the U.S. to get quotes for geothermal systems in the $40,000-$60,000 range for a typical home. I’m just a poor blogger/farmer/pretend engineer, so that’s too pricey for my blood.

Using a mini-split for a geothermal system is definitely going outside of the design of the unit, but that is the challenge of the day and we’ll need to engineer our way through it. What could go wrong? First, we need to change how this unit operates. Mini-splits are air-to-air heat exchangers. To function as a ground source heat pump, this will have to turn into an air-to-liquid heat exchanger. We’ll also need to solve how to get the fluid in and out of the ground without freezing, and how to monitor the system to collect data for science!

Design Concepts

The mini-split we chose is a cheap one off of Ebay. It is a 28,000 BTU (~ 2 ton) unit with dual zones, each with 12,000 BTU heads. We could have gone with two mini-splits, but that would require two tanks (which are expensive), or one large tank (very expensive) to house the two sets of condenser coils. So a dual zone unit seemed like the best choice, even if it reduces redundancy.

To modify the mini split to be a GSHP, we will have to disassemble the brand new unit (and void the warranty of course), extract the condenser coil and then either buy a tank or build a tank for it to sit in. In the tank we will have the geothermal fluid (distilled water and methanol), sensors to monitor temperature of the coils and the fluid, an inlet and outlet to the ground loops, and a pump to circulate the fluid.

Ground Loops

It is recommended that the ground loops be 600 feet of tubing per 12000 BTUs. So we will run two ground loops of six hundred feet each, plus the return line which will add about another on hundred and twenty feet to each loop. I decided to use 3/4″ PEX as my line because of cost and availability. The pump output will go to a splitter to divide the flow (hopefully equally) between the two loops. This will halve the flow rate through each loop, giving the fluid more time to cool and also reducing flow-related pressure loss (friction) in the pipe.

Data Collection

The secondary goal of collecting data will be accomplished by a vast network of sensors. The goal is to have ground temperature sensors and water temperature sensors placed along the ground loops. There will also be vertical and horizontal arrays of sensors to see how the addition of a geothermal system effects the ground temperature both above the loops and to the side of the loops. A set of control sensors will be used to take the normal ground temperature out side of the geothermal area. This will be combined with a weather station and several temperature and humidity sensors both inside and outside the building to build a complete picture. We should also be able to calculate the coefficient of performance (C.O.P.) in real time.

Other System Components

For this system to function properly, there will be a variety of other components that are not explicitly stated in this post. We will cover all of the details of each component in future posts. You can expect us to talk about pumps, flow rates, pressure loss, and automation of these components as well as how we installed them. We will also cover the science and theory behind all of this, so that you may understand better, what happens out of sight.

Thank You

Thank you for reading my post. If you find that you gained some knowledge and would like to read more, please consider giving us a like and subscribing. It’s free and it helps us get sponsors for future content. Also if you feel we are worthy of a small donation, you can leave a tip at the button below. Every little bit helps as the projects can be quite costly.