About 2% of new commercial buildings in North America in the last few years have been built with a GCHP system, and few building managers have had the opportunity to operate a building with a GHX. What are these pipes coming in from the ground around the building? What do they do? What can I expect from them? How do I control the temperature of the heat transfer fluid coming from the ground?
The most important thing to understand about a GHX is that it is not in infinite energy source. It is not like a gas line that supplies an infinite amount of energy (as long as the gas bill is paid) limited only by the size of the pipe connecting the building, or a cooling tower with a heat dissipation capacity based on the maximum temperature and humidity for a location. A GHX is connected to a finite mass of earth and rock, and the capacity to absorb heat or give up heat is limited by the thermal properties of the earth, the mass, the effectiveness of the heat exchanger connected to it and the temperature parameters of the heat pumps.
It is much more like a rechargeable battery rather than an infinite energy source / sink. As heat is extracted from the GHX the temperature of the fluid will drop. When heat is rejected to it, the temperature will increase. As we cycle through the seasons, the temperature drops as we go through winter and increases as we progress through spring and summer. How much it rises and falls depends on the capacity of the earth around the pipe to absorb or give up thermal energy.
With reasonably accurate energy modeling of the building, and a good understanding of the dirt and rock the heat exchanger is built in, it is possible to predict the temperature we should see from the GHX on an hourly and annual basis, based on average weather. A system designer should provide you, as the building operator, information showing the temperatures you should expect to see from the GHX throughout the year and the temperature range you would see on a day to day basis.
By monitoring the temperature of the GHX, even you are only recording the temperature coming from and returning to the ground once a day, you can establish a trend of what the temperature should be at a given time. If the temperature deviates from the expected temperatures by any significant amount, there may be things happening in the building that weren't considered in the energy model or building use may have changed. If you can find out what is causing unexpected changes you can make changes to the building and/or system to get the GHX back on track.
GEOptimize can work with you and your designer to help you set up a program to monitor the operation of your system.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.
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