Determining the equipment capacity for a conventional HVAC system is relatively simple. You only need to calculate the peak heating and cooling loads of the building if the building is connected to the gas grid. Looking up the size of the gas pipe needed to deliver enough energy to the building is as simple as looking it up on a chart supplied by the gas company. Selecting a cooling tower or air cooled chiller can be done from a catalog or supplier's website. There's more work involved in designing the energy source / heat sink for a GCHP system.
It's not much different than the calculations engineers working for a utility have to do. Someone has to determine how much gas has to be pumped down a gas pipeline to keep all their customers warm through the winter. And they need to ensure enough gas is extracted from the enough gas wells to maintain enough pressure for furnaces and boilers to operate properly. In addition to that, someone has to determine if there's enough gas in the ground that the wells are tapping into to last through the winter.
The job of the GHX designer is really not much different than that...only on a smaller scale...on a building scale. The designer needs to calculate with reasonable accuracy that there's enough pipe in the ground to be able to extract enough energy to keep the building warm. He or she has to be sure enough of the geology on the site to know the energy can move through the ground to the heat transfer fluid in the pipe to meet the peak heating demand.
Then the calculations have to be done all over again, but in reverse. Can the energy taken from the building to keep it cool be pushed back into the ground to ensure the heat transfer fluid stays cool enough for the heat pumps to operate efficiently through the summer.
The calculations need to be more detailed and more accurate than they do if the building is simply connected to the infinite energy supply of the grid. And then, to make it a bit more complex, the designer has to consider the long term impact of the energy loads. If more energy is pushed into the ground every year than is extracted, how can overheating of the ground be prevented. Should the pipes be spaced further apart? Would it be more cost-effective to integrate an auxiliary cooling device? Or will the building extract more heat than it injects to the ground. Is auxiliary heat needed?
And then, to reduce construction cost and/or lower energy costs, are there things that can be done to the building construction or building systems to reduce the size of the GHX? To design an efficient, cost-effective system takes more calculations and thought than designing a conventional HVAC system. On top of all that, the designer needs to ensure the system is installed as designed.
Seen from this perspective, it's clear that designing a cost-effective and efficient GCHP system requires the same thought as designing a utility for a building.
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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