Geothermal Rescue – Open Loop Water Turns Acidic
This geothermal rescue project involved an open loop geothermal system, corrosive water, and saving multiple heat pumps from premature failure. Open loop systems use well water to convey the Sun’s energy from the Earth to the geothermal heat pumps inside. Because heat pumps need up to 3 gallons per minute per ton of capacity, the required flow of water can quickly become quite high, especially when the heating or cooling load is high. For example, if the average geothermal installation is 10 tons of cooling or heating capacity, that would mean a flow of 30 gallons a minute for every minute the heat pumps were on providing heating or cooling. A normal potable well will produce 15 gallons a minute, so needing 30 gallons per minute for days on end is a prodigious amount of water.
This is the primary reason that open loop geothermal systems are fairly rare and that most people that install geothermal systems that are closed loop that do not use potable water.
We have seen that the removal of large amounts of water from the water aquifer by an open loop geothermal heat pump system can often lead to water quality changes – most often the water becomes more acidic. That happened in this geothermal system – the heat pumps were going off on various faults all tied to inadequate flow – we took apart the main loop circulator pump and found that the impeller and shaft inside the circulator had been dissolved! We tested the water and found that it was very acidic at pH 5! This was very dangerous to the heat pumps and piping – any leaks that might develop would NOT be covered by the manufacturer’s warranty. As we disassembled more of the system, we discovered more parts had been eaten away – such as this metering assembly.
We developed a plan for the homeowner – we would install a large plate and frame heat exchanger to separate the outside water (which was now corrosive and acidic) from the clean inside geothermal water that served the heat pumps while still transferring heat between the two fluids. Here we can see the stack of stainless steel plates that will be tightly squeezed between the two plates when the nuts are tightened – each alternating plate of stainless steel has inside water, the other plates have outside water.
This project required some reworking of the mechanical room to accomodate the new heat exchanger. Dave Sprague spent considerable time laying out where the heat exchanger would go.
Dave carefully located the anchor points and the heat exchanger’s back plate was installed. Each plate weighed 200 pounds.
The stainless steel inner plates were assembled and the front plate tightened up to the correct torque specs. Based on the water quality, we would expect to disassemble this heat exchanger every other year to scrub the individual stainless steel plates with a nylon bristle brush and water. Stainless steel was chosen for its resistance to the corrosive outside water.
The existing geothermal piping was reworked so that we could install the heat exchanger as the separator between the outside water (copper pipes on the left side) and the inside loop water (copper pipes on the right side). The large horizontal ball valve in the upper part of the picture is a bypass which we use when cleaning is necessary.
The final installation included a chemical feeder to neutralize the inside water until all traces of outside corrosive water were removed. The installation of this plate and frame heat exchanger converted this failing open loop geothermal system into a reliable hybrid open/closed loop geothermal system. The corrosive water caused by the change in the aquifer would no longer be able to damage the inside geothermal heat pumps.
Our goal is to be Martha’s Vineyard’s premier plumbing and HVAC shop – we appreciate the opportunity to show you some of our work.