Real Time Data on Actual Geothermal COPs – Part 2 of Lessons Learned from 100,000 Hours+ of Real Time Geothermal Monitoring Data
This is a very nervous time for the geothermal industry and I can feel it when I’m on the phone with installers. There’s a lot of geothermal companies that have been around for 20+ years and now there’s a lot of innovation happening around policy, real time monitoring, and playing around with different ground loop heat exchangers, that industry veterans haven’t had to deal with before and it’s uncomfortable for them.
Here’s my main message to the geothermal industry: if you’re asking everyone to look at new ways to heat and cool buildings, you also need to be looking at new technologies, policies, and ways of doing things yourselves. Some of you are, but on average I notice more hesitancy within the industry than excitement about embracing (inevitable) change. I could be wrong on this, just my perception.
Monitoring projects means that we’re going to find great, good, and bad systems. In fact that’s the goal. Matt is a little more soft spoken so he wouldn’t say this, but yes, we will find bad systems.
With that being said, I’m super excited to be working with Matt and publishing this data. There’s only a small group of things the geothermal industry can do to proactively (unlike waiting around for fuel prices to increase by 300%) increase the adoption of the technology; including working for better policy, better ground loop innovations, decreasing customer acquisition costs, and verification of system performance.
Geothermal technology has a low margin of error, if anything is messed up during the design, installation and (most overlooked) operation of the system, the efficiencies will be shot. Here’s some of the interesting bits of information that can easily be found monitoring more than just kWh of the system:
How much does the COP drop due to pumping losses
Equipment failures; low refrigerate, valves, and pumps.
If you want more on monitoring check out these previous resource:
Performance Based Contracts Are the Future of Geothermal
Lessons Learned about Ground Loop Sizing from 100,000+ Hours of Real Time Geothermal Monitoring
Free Course: How to Install, Commission and Use Real Time Geothermal Monitoring
Matt and Ground Energy Support’s data is a robust but small sample size, compared to the whole industry. But it shows the possibilities of what we can figure out with a larger sample size and more data. If you’re a manufacture, distributor, installer or driller, utility, or state energy official and would like to go a monitoring and verification study. Feel free to call me, Chris Williams at 800 393 2044 ex. 33, or Matt Davis at (603) 867-9762
Future posts will include
Geo Monitoring Quality over Quantity: Why All Data is Not the Same
Characteristics of the Best Monitoring and Verification Study
How Monitoring can Eliminate, or Prevent, Angry Customers Once and For All
Expected Versus Actual Performance: Comparing LoopLink Predictions to GES Data
Enter Matt Davis from Ground Energy Support
This article is the second in a three-part series prepared by Ground Energy Support LLC (GES) that will highlight some of the lessons learned from over 100,000 hours of real-time ground source heat pump (GSHP) monitoring data. This is written for GSHP installers of residential and light commercial systems who want to learn how to leverage real-time Performance Monitoring to build better GSHP systems, reduce their risk and callbacks, and ensure customer satisfaction.
The previous article focused on showing how data from real-time monitoring can be used to assess how the ground loop is performing relative to both the installed capacity and the measured heating load. The daily load profiles presented in that article are now available to GxTracker™ users as part of their Performance screens (login required to view Load Profile graph).
This article focuses on the overall system performance, factors that affect performance, how real-time monitoring can be used to evaluate performance of installed equipment, and identify and address issues before they become problems.
The number of sites represents a relatively small sample size and the different measurement techniques used at the sites result in different levels of accuracy. In spite of these limitations, we are able to identify and illustrate several factors that affect system performance. As more sites are instrumented with web-based monitoring systems that have the capability of quantifying ground loop performance, geoexchange, and electrical consumption, there will be an unprecedented ability to assess a wide range of GSHP designs over a wide range of geologic and climatic conditions.
Measuring GSHP System Performance
GSHP systems have the potential to significantly reduce the energy needs for space heating and cooling (an estimated 4 Quadrillion BTUS in the US) and reduce greenhouse gas emissions. However, these benefits will only be realized if the systems operate efficiently. Efficient operation requires proper design, installation, and maintenance.
One common method for assessing system performance of residential systems is to review the customer’s electric bills on a regular basis. While lower electric bills are a clear indication that the systems are operating efficiently, it is often difficult to parse out other factors that contribute to the overall electrical consumption. Additional challenges to using a customer’s electric bill to assess system performance include: new construction has no baseline for comparison; retrofits will often combine the addition of a heat pump with other efficiency measures, making it difficult to attribute the savings to the GSHP; and finally, when an electric bill is higher than expected, the cause may be unrelated to the GSHP system. One of the biggest disadvantages of relying on electric bills as a measure of system performance is that the homeowner is left to identify and report GSHP performance problems. In short, the most effective way to demonstrate that a GSHP system is performing efficiently, and thus ensure customer satisfaction, is to monitor the system in real-time so as to assess key performance metrics.
Many factors contribute to the overall performance of a GSHP system. Through our monitoring activities to date, we have found the main factors to be:
Heat pump performance and ground loop temperature
Pumping energy necessary to circulate loop fluid
Heat pump cycling patterns
Auxiliary electric heat
One of the key metrics of assessing how a GSHP system is performing in heating mode is the Coefficient of Performance (COP). The COP is the ratio of useable thermal energy to the thermal equivalent of the electricity used to operate the system.
While measuring the COP is not always the primary objective, all of our real-time monitoring systems include, at a minimum, measurements of entering and leaving water temperature (EWT and LWT), heat pump status, and an estimate of fluid flow. When power measurements are not available, power consumption is modeled using the heat pump specifications, a pump penalty based on type of system and flowrate, and measured runtimes. These methods provide representative values of COP with an error of +/- 20% and are intended to show overall trends in system behavior.
In some installations, a higher level of accuracy is desired and additional measures of power consumption and, if necessary, flowrate can be included. When accurate measures of both flow and power consumption are available, the accuracy of the COP improves to approximately +/-5%.
Because the accuracy of the computed COP varies from site to site and quality assurance measures are necessary to insure the values are representative, COPs are not computed automatically as part of our performance metrics. Rather, the necessary components are available by downloading either the minute-resolution observations or integrated daily values.
Daily Average System COPs
The Daily COP values presented in Figure 1 and summarized in Table 1 illustrate some of the main factors that affect performance. Overall, there is an upward trend in performance as heat pump technology has gone from single stage to multi-stage compressors.
The system with the lowest performance (Site A) has single stage heat pumps using a standing column well. The two-stage heat pump on a standing column well (Site B) has a higher COP, but is adversely affected by the power required for fluid pumping. The COP of the two-stage system on the closed vertical loop (Site C) varies more through the year due to both the change in EWT and the use of the auxiliary electrical heat in late February. The performance of the two-stage system using two well groundwater loop (Site D) is comparable to, and slightly higher than, the other two-stage heat pump systems. The system with a variable stage heat pump (Site E) has the highest average COP. This is likely due to a combination of the variable-stage technology and its geographic location which enabled it to maintain higher EWTs throughout the winter.
These COP values are consistent with those reported by Puttagunta and others in their 2010 study of residential GSHPs. We have also found that, while the in-field performance is typically less than the AHRI rating, the systems are seen to provide a reliable technology for heating and cooling at consistently lower cost than conventional systems.
Table 1: Seasonal Average System COPs, Winter 2012-2013
Standing Column Well, 2 Heat Pumps, Single Stage
Standing Column Well, 1 Heat Pump, Two Stage
Closed Vertical Loop, 1 Heat Pump, Two Stage
Open Two Well System, 1 Heat Pump, Two Stage
Closed Horizontal Loop, 2 Heat Pumps, Variable Stage
Factors that affect System COP
There are several factors that affect the overall GSHP system performance. Because of the wide range in GSHP system designs and equipment available, there is not a single best approach to optimizing performance. Rather, we show how monitoring can help characterize the elements that affect specific installations. Again, the lessons learned and examples presented in this initial series of articles focuses on heating mode and uses examples from installations ranging from US Climate Zones 4 to 1.
Heat Pump Cycling
It has long been recognized that rapid cycling of heat pumps diminishes performance and increases mechanical wear on the equipment. In recent years, more efficient dual stage heat pumps have become common that enable heat pumps to operate for longer cycles, produce lower heat output, and use less electricity.
Figure 2 shows the heat pump cycle patterns for three different systems: one with two single-stage heat pumps, a two-stage heat pump, and one with a variable-stage heat pump. While the single-stage heat pumps reach their expected heat of extraction (~22MBtuH for the 3 ton and ~40MBtuH for the 5 ton), the average production of the heat pumps (while running) over this same period was 17 MBtuH and 32 MBtuH, about 20% below their steady state production. In contrast, the two-stage heat pump is able to maintain its steady state heat production in part load for cycles of approximately 30 minutes and then, when additional heat is necessary, switch into full load. With two-stage heat pumps there is much less efficiency lost from cycling on and off. The final example is a new variable stage heat pump that operates over extended periods of time, adjusting the heat output to meet demand. In general, the lower stages provide greater performance by varying heat output over multiple stages thereby increasing overall performance.
Entering water temperature is well-recognized as one of the leading factors affecting system performance and heat pump manufacturers provide performance specifications for temperature ranging from 25 to 100 ° F.
In the example shown below (Case A from last week’s article on Loop Performance), the power consumption of the heat pump circuit (which includes the flow center), and the Auxiliary electric heat are being monitored separately with a WattNode power meter. The COP is calculated for 15-minute intervals through the 2012-2013 winter. To remove the variability of performance associated with the heat pump cycling on and off, we only include 15-minute intervals for which the heat pump was running continuously. The EWT ranges from 33 to 51 °F. While there is an overall correlation between COP and EWT, there is also considerable variability at a given temperature.