Every once in a while a developer comes up with software that makes life a lot easier! Ground Loop Design (GLD) has just done that for the commercial ground source heat pump industry. Let me explain:
Designing a ground heat exchanger (GHX) is actually a two stage process. The first stage involves determining how much pipe to stick in the ground based on the land area available to install it, the geology and availability of contractors and equipment to install it the way it’s designed. Over the last few decades, a number of software packages have been developed to make the calculations needed to determine how much pipe is needed…the first stage of the design.
The first stage of GHX design has been fairly well served for the last couple of decades by a number of software packages. While some of the packages are capable of determining only the depth and spacing of boreholes required in a vertical GHX, others can calculate vertical and horizontal GHX lengths. One calculates the amount of pipe required in a lake heat exchanger as well. The ability of the software to predict future GHX operations is largely dependent on the theory utilized by the software as well as the accuracy of the information input by the designer. And of course software ease of use varies from vendor to vendor based on the quality of the UI (User Interface).
The second stage of the design…not so much!
Till now it has been ignored both by software vendors and unfortunately, by many designers. The result is a lot of poorly performing systems that have been and continue to hurt the reputation of the industry. The configuration and layout of the boreholes or trenches, detailed design of buried headers, number of circuits per header, runout pipe sizes, flushing flow rates, specifications for heat transfer fluid and several other details directly impact overall system efficiency and, in some cases, determine if it will operate at all.
The first design stage is equivalent to designing the engine…the second stage is like designing the transmission. You may have the power, but no means of getting it to where it’s needed. A system needs both.
Until now, few tools have been available to engineers and GHX designers to optimize this second design stage and take full advantage of all the pipe buried in the ground. There are a few charts and graphs available from the fine folks at the International Ground Source Heat Pump Association (IGSHPA), the American Society for Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) and a few other organizations, equipment suppliers and even pipe manufacturers. More progressive engineers and designers working on these systems have developed their own static spreadsheets to help them optimize their systems...but most authors consider them proprietary, making them unavailable to the rest of the industry.
Based on projects I’ve seen and heard about, there are quite a few projects out there that don’t work as well as they could. In large part, this is because truly good, easy to use design tools for this stage of the design have not been available.
There are projects operating with pumps that are as much as 10 times larger than they should be…because the pressure drop through the system is too high. Not only are owners paying too much to circulate the fluid, a parasitic load that reduces overall system efficiency, but the over-sized circulation pumps add heat to a GHX that is already too hot for the heat pumps to cool the building efficiently and in some projects are a primary cause of long term temperature degradation.
There are projects where 20% to 30% of the pipes in the boreholes are blocked by air trapped. With no flow they contribute nothing to the system! This is because the contractor couldn’t find a flush pump large enough to flush the air from the system. In other words, the owner paid for a lot of expensive heat exchanger they can’t use and their system is a lot less efficient than it should be.
There are projects where the heat transfer is compromised in the heating season because the flow through the GHX circuits is too low. An unfortunate combination of low temperatures, high antifreeze concentration, circuit numbers and / or pipe size causes laminar flow in the GHX. Such systems quit working completely when the fluid temperature drops too low.
The importance of the detailed configuration and layout of a GHX cannot be over-emphasized. Designing a GHX requires attention to a number of details…none of them all that difficult...but there’s quite a few of them…and an accumulation of errors will seriously compromise a system.
Ground Loop Design (GLD), one of the software vendors, has had a powerful computational fluid dynamics module integrated since their 2010 release. The module allows a designer to easily check all those details. It allows you to check flow rates, Reynolds numbers, pressure drop and the like when the system is in operation as well as when the system is being flushed. It auto-optimizes headers and even calculates the volume of fluid in the GHX.
Unfortunately, few people use it. I think this is because first, people don’t recognize the importance of piping design; second, because some people don’t know about it; and third, because some find the user interface to be about as user friendly as a rattlesnake! I am probably one of the few geo geeks that took the time to read the extensive user manual (I had several hours to kill on a long flight and I was bored, OK!) and have taken full advantage of the tools powerful and great features.
New User Interface
The folks at GLD have spent the last three years developing a user interface that finally makes this powerful tool very easy to use. It takes full advantage of the powerful fluid dynamics calculation engine, allowing a user to ensure that flow rate through each GHX circuit, with the specified fluid, will not be in laminar flow while automatically minimizing the pump power necessary for efficient system operation.
The fluid dynamics module is well integrated with the vertical, horizontal and surface water heat exchanger design modules used to calculate the amount of pipe needed for a system. GHX design parameters can be imported directly into the fluid dynamics module to ensure your system will be easy to flush the air out of, transfer heat to and from the ground under all operating conditions, and optimize the size of the pump for maximum efficiency.
Something I’ve found most interesting with this new tool is the fact that, even with a perfectly designed reverse return header system, the flow rates through each of the GHX circuits connected to the header are not identical…the circuits in the middle of the header can have a little less flow, and lower Reynolds numbers, than the circuits at the beginning or end of the header. In other words, it’s possible for three or four circuits in a ten circuit header to have Reynolds numbers lower than anticipated, reducing heat transfer which in effect results in under-utilization of a large percentage of the GHX your client has paid for. In extreme situations, the temperature of the fluid delivered to the heat pumps will drop to the point where the heat pumps can’t work.
I saw the effect of this in an ice rink project I worked on in 1989. Our singular goal was to reduce pumping costs from the ice rink floor. We designed a system to reduce pumping power by about 50% compared to a conventional rink floor design. Not knowing at the time that the center circuits would not get the same flow rate as the end circuits, we were able to make ice from the ends of the rink to about the blue lines…but not between the blue lines! It’s funny in retrospect…not so much at the time! It did eliminate many of the fights between the opposing teams though! With this software we would have been able to predict and avoid the problem altogether.
Using the new GLD tool cuts my design time by probably 90% compared to my old way of doing things (using a spreadsheet calculator that I made myself). It literally only took about 5 minutes to design and optimize the flow characteristics of a 24 circuit GHX. I ran about have a dozen iterations using different numbers of circuits, different flow rates, different fluids and each took less than a minute. And with each of the alternatives the software showed me how much pipe of each size and type was needed, fluid volume, flow and pressure drop needed to flush the system, flow rate and pressure drop while operating, Reynolds number range with the selected fluid as well as estimated pumping power for each design. If I wanted to dig into the details of the system, it was easy to check flow rates, pressure drops, Reynolds numbers, etc. through any section of the system. If one borehole is 30’ longer than the others and another is 20’ shorter than the others, it’s easy to customize the GHX design to match the system as built and check the impact on flows and Reynolds numbers. Or if one runout pair is twice as long as the others, it’s easy to determine if the best solution is to add balancing valves, or if it’s better to bump the pipe size of the runout pairs up from 2” to 3”.
There really is no comparison between the old way of doing things (with charts/graphs/spreadsheets) and the new way with GLD2016 for calculating flow rates, checking Reynolds numbers, pressure drops and flushing flow rates.. The software allowed me to consider half a dozen alternatives with different pipe sizes, different borehole spacing, different fluids, different header configurations, etc. to quickly find the most efficient and cost-effective layout and configuration for a specific project…in a few minutes.
If you already have GLD, the new upgrade is well worth the money. If you use other GHX design software, the fluid dynamics module will apparently be available as a standalone software package. Either way, it does for the layout and configuration of a GHX what loop length software did for the industry 15 or 20 years ago. This is, in many respects, game changing software for the ground source heat pump industry and well worth your while if you want to work in this industry.
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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