I've often seen a tender for construction of a GHX that specifies the GHX must be designed to supply "xxx" Btu's or kWh of energy based on a peak cooling load of "xx" tons or kW and a peak heating load of "xx" Btu/h or kW. In most cases little additional information is provided - not the area of the building, what the building is used for, how it's constructed... nothing else. Designing a GHX based on that information is impossible.
To design a system that works predictably, it's critical that the designer has a good understanding of the building loads. How is the building being built? What are the insulation values? What kind of glass has been specified? How is fresh air being introduced to the building? What kind of lights are being specified, how many are there and how many hours per day will they be turned on? Are the controlled with occupancy sensors? How many people will using different areas of the building? What will they be doing and how long will they be doing it?
This information is critical to determine what the peak heating and cooling loads are, and more importantly, how how much energy they are taking from the building each hour of the year, month and a total for the year.
To optimize the design of a GHX it is absolutely worthwhile working with the architect, mechanical engineer, electrical engineer and other stakeholders in the design and construction of the building...they have the ability to make significant changes to the heating and cooling loads of the building as well as how much energy the system needs to provide to the building or remove from the building. That changes the energy balance and allows you to take much greater advantage of a smaller, less expensive GHX.
Think about a GHX as a bank account. If you take $100 out of your bank account every week, and no money is deposited, eventually the account will be overdrawn. A GHX is no different...if you take more energy than is put into it...it will eventually get to cold for the heat pumps connected to it to work efficiently, or they even quit working. The same thing happens when more energy is rejected to the GHX...eventually it will get too warm for the heat pumps to work efficiently.
By working with the owner and design team and adjusting a detailed energy model of the building as changes are made to the building or building systems, the energy balance of the building changes. Changing the glass specifications changes the solar gains to the building...more or less cooling is needed. Changing the lighting specifications changes the internal gains from lights. Changing how fresh air is introduced to the building...recovering heat from the ventilation air, or specifying a CO2 sensor to control the ventilation air based on demand, changes the amount of heat loss or gain to the building to condition the fresh air.
Balancing the energy loads to and from the ground by adjusting the building loads will make a significant difference on the size of the GHX. The GHX will rely less on heat transfer from the rock and soil surrounding the GHX or on solar gains during the summer, or rain infiltration or a fortuitous aquifer to wash away excess heat that's been rejected to the GHX. Rather, it will perform much more predictably based on the energy flow into and out of the ground from the building.
The number of boreholes or trenches can be reduced and the spacing between the boreholes or trenches can be reduced. The cost of building the GHX and the land needed to build it are both reduced. And reducing the cost of the GHX makes the return on investment the owner makes in a GCHP system much quicker.
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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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