closed or open loop geothermal- NCARB practice question
For the question below, why is vertical open loop not the correct choice? It is more economical for the condition mentioned in question:
An architect is specifying a geo-exchange heat pump system for an office building. The building is on a site that has a significant concentration of groundwater and is conducive to well drilling. The owner wants the heat pump system that is the most cost-effective and energy-efficient.
Which heat pump system should the architect specify?
CORRECT RESPONSE
Vertical closed loop
For ground that is conducive to well drilling, the deep ground will be warmer in the winter and cooler in the summer than ground closer to the surface. Vertical systems are most efficient in ground that is conducive to well drilling. Closed loop systems use much less pumping energy than open loop systems.
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Page 605 from heating cooling and lighting states " A closed vertical loop is usually preferred because much less pumping energy is required", this statement compared both vertical loops, I assume that the correct answer was based on this statement... however there is no mention of which is most cost effective out of both system....or why less pumping energy is required....based on heating cooling and lighting (primary resource) vertical closed loop are the better choice over vertical open loop.
But I agree with Prof. Ermann,it would be great to hear from a geothermal heat pump expert.
Just doing a quick chatGPT :
"vertical closed-loop systems, the fluid is circulated in a closed circuit with less head pressure and lower flow rates, consuming less energy for pumping. "
It seems that closed loop systems although cost a bit more outperforms vertical open loop systems making it more energy efficient which is one of the keywords in the problem..
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Did you mean vertical “closed” loop or vertical “open” loop? It might not be the correct answer because the problem states explicitly that there is a significant amount of ground water. This would imply, to use an Open System. It is my understanding that vertical geo heat pumps do not necessarily need a source of ground water to operate. They can help, but are not needed. I’m studying for my PPD now. But it is quite confusing because surely closed systems are more cost effective from an energy standpoint than an open system!
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All,
I think I can give a little insight on this - I'm a little less clear on long-term cost-effectiveness, but I can definitely speak to the energy-efficiency component of the question.
A closed-loop system circulates the fluid in a closed loop. That means that the pump can push the geothermal fluid (usually water with a little antifreeze) down into the well with the aid of gravity, which will push an equal amount of fluid back up - the whole system is basically at equilibrium, so it takes relatifely little effort to circulate the fluid.
An open-loop system pulls water out of a well, and usually just dumps it back down a second well or otherwise discharges it to daylight or back down the original well - this means three things. One, you may need to drill two wells, which is an increased cost. Two, the pump has to pump the water all the way up from the bottom of the well against gravity, without the assistance of a closed return system, requiring the pump to do more work - which uses more energy and costs more. Three, if a second well isn't used, and the water is discharged back down the same well, the returned water will mix with the water in the well. If you do this, you have changed the temperature in the well in a way that will negatively effect the system's efficiency.
The point is that even if there is a lot of groundwater available, it should be used as a heat sink for a closed-loop system rather than be used as the source of heat/cool itself in an open loop system.
Here's a little further reading:
https://dandelionenergy.com/open-loop-vs-closed-loop-geothermal-systems
Best,
Ralph, the Amber Book Team -
One more time! Michael and I discussed this, he's reaching out to an HVAC engineer.
For the site, a vertical system is definitely better than a horizontal system - you definitely want to get access to the groundwater for heat exchange, and a well to run vertical pipes in is the way to do that.
Whether the open-loop or closed-loop system is most cost-effective and most energy-efficient will depend on the groundwater's temperature and what disposal options are available for the "waste" groundwater once it has exchanged heat with the heat pump.
An open-loop system definitely uses more pumping energy than a closed-loop system for the same Delta-T. But, pumping energy is only one component, and an open-loop system may be able to achieve a better Delta-T for the available site than a closed-loop system ever could.
Geothermal heat pumps work best in climates with cold winters and hot summers - in summer, you slowly heat up the ground with the excess heat from the system, in winter, you slowly pull that heat back out. In more extreme climates without multiple seasons, a closed-loop geothermal system will slowly permanently heat up or cool down the ground under the building as there isn't a balanced need throughout the year. So, a closed-loop system in a more extreme climate will lose efficiency over time as it has a worse and worse Delta-T available, making an open-loop system a much better option if an adequate water source is available.
Gilmo definitely found the excerpt of Heating, Cooling, and Lighting around which this question is based.
Here is some further reading on open-loop systems, here is a discussion of the benefits and drawbacks of the two systems from an HVAC installer (they say open loop systems are generally more efficient and have lower install costs but higher maintenance costs).
Point being, the more we look at it, Michael and I suspect an HVAC engineer would actually say "it depends" for the criteria given in the question. It depends on the project's climate, what temperature the groundwater is, and whether the site has some means of accepting the return water from an open-loop system.
Best,
Ralph, the Amber Book Team
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