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Performance analysis of a ground-source heat pump system using mine water as heat sink and source.

INTRODUCTION

Development and deployment of clean energy technologies has become one of the forefront agenda items in both developed and developing countries to strengthen the economy, protect the environment, and reduce dependence on foreign oil (NSF 2014; EERE 2015a, b; Gallagher 2014; CEM 2014). Geothermal technologies are among the potential clean energy technologies developed for various applications ranging from generating electricity using high-grade heat from deep subsurface sources to provide space heating, cooling, and hot water using the nearly constant temperature of the earth's crust as both heat sink and source (ASHRAE 2011). For the latter, heat transfer between the building and the ground is made possible using a heat pump and a system of pipes buried in the shallow ground, hence the name ground-source heat pump (GSHP) system. Through innovative design and configuration of the ground heat exchanger, a variety of heat sink/sources have been explored, depending on the hydrogeological features of the site such as standing column wells, lakes/ponds, municipal wastewater, sewage effluent, foundation/ pilings, underground mines, and a combination of various ground and water sources (Goetzler et al. 2012; Sachs et al. 1998). One such innovative design of the GSHP system using flooded-mine water as a heat sink and source is implemented at a 56,000 [ft.sup.2] (5203 [m.sup.2]) newly constructed research facility in Butte, MT.

The use of mines is a promising option for geothermal energy recovery (Ghomshei and Meech 2003; Wazlaf and Ackman 2006) as a heat source, a heat sink, or both. Its potential lies in the use of otherwise unexploited resources--abandoned and flooded mines. However, such application has not been extensively studied as for other conventional GSHP systems (Hall et al. 2011). An investigation of about 1600 abandoned mines in the United States for prospects for ongoing discharge of useful quantities of warm water resulted in a detailed look at 80 sites (Lawson and Sonderegger 1978).

This project is one of the 26 GSHP projects, which were competitively selected under the 2009 American Recovery and Reinvestment Act (ARRA) to demonstrate the benefits of innovative technologies for reducing the cost and/or improving the performance of GSHP systems. The installed mine-water GSHP system uses two abandoned and flooded underground mines located approximately 900 ft (274 m) west of the building that is being heated or cooled. These two mines are connected underwater, as indicated by the same water levels in both mine shafts, and considered as a single underground reservoir. The water level in the mines is 110-120 ft (33.5-36.6 m) below the ground surface. The mine-water temperature is about 78[degrees]F (25[degrees]C). Nearly 20 years of continuous pumping at a nearby mine has demonstrated that mine-water temperatures are stable and ample heat is available for long-term use (Thornton et al. 2013).

A set of 6 in. (0.152 m) supply and return pipes was installed from the building to the mine. These pipes were branched out with 3 in. (0.076 m) high-density polyethylene pipe (HDPE) pipes to the 100 ft (30.5 m) underground level, where they were connected with 20 %in. (19 mm) 600 ft (183 m) long HDPE parallel loops. These loops were installed through the airway of a mine shaft and down an abandoned hoist way, then immersed into the mine water, which is 200 ft (61m) below the mine entrance, as shown in Figure 1. Such an installation was invented for this project and had not been done before. The closed-loop piping system was selected instead of a probably cheaper open-loop system because the grantee does not have the water rights and was, under state law, not allowed to pump water out and back into the mine shaft. Even if the grantee is allowed to pipe the water out--and then turn around and pump it back in the same hole--the grantee would have to treat the water to meet clean water standards.

Two redundant parallel 7.5 hp constant-speed pumps are used alternatively in a lead and lag fashion to circulate water in the closed loop immersed in the mine water (referred as mine-water loop). The pumps shut down with the heat pump when the outside air temperature is above freezing. Freezing may occur as the pipe enters the airway before it goes down into the mine, where it is warm. For freeze protection, the pump may run continuously even when the heat pump was turned off.

The GSHP system works in conjunction with an existing 6200 kBtu/h (1817 kW) heating system, which uses steam produced at a central plant to make hot water at the building, and a 170 ton (595 kW) air-cooled chiller to provide space heating and cooling to the building. As Figure 2 shows, a 50 ton (175 kW) water-to-water heat pump, which has two 25 ton (87.5 kW) scroll compressors, is connected with the existing hot-water and chilled-water piping in the building. The heat pump can operate in heating or cooling mode based on the outdoor air temperature, as described below:

* It operates in heating mode when the outdoor air (OA) temperature is equal to or below 60[degrees]F (15.6[degrees]C). In this case, the valves modulate to divert part of the return water of the existing building hot-water loop to the load side of the heat pump and isolate the connection to the existing building chilled-water loop. When a substantial flow rate is detected, the heat pump starts to heat up the return water and the heated water then goes back to the building hot-water loop before entering the steam heat exchanger (HX). The steam HX then adds the remainder of the heat, if it is needed, to maintain the return temperature from the building heating water loop at a setpoint according to a reset schedule--135[degrees]F at -10[degrees]F (57.2[degrees]C at -23[degrees]C) OA temperature, 100[degrees]F at 60[degrees]F (37.8[degrees]C at 15.6[degrees]C) OA temperature. The remainder of the building hot-water distribution system and the heating terminals operate as originally designed during GSHP system operation.

* It operates in cooling mode when the OA temperature is above 60[degrees]F (15.6[degrees]C). In this case, the valves modulate to divert part of the return water from the building chilled-water loop to the load side of the heat pump and isolate the connection to the building hot-water loop. When a substantial flow rate is detected, the heat pump starts and intercepts the chilled water back to the building chilled-water loop before it returns to the chiller. The chiller then provides the remainder of the cooling, if it is needed, to maintain the chilled-water supply temperature setting of 45[degrees]F to 65[degrees]F (7.2[degrees]C to 18.3[degrees]C). The remainder of the building chilled-water distribution system and the air-handling units operate as originally designed during the GSHP system operation.

Also shown in Figure 2 is the location and name of data collection points available from the demonstration site, including the supply- and return-water temperatures in the mine-water loop (temperature in mine-water supply [TMWS] and temperature in mine-water return [TMWR], respectively), the flow rate in the mine-water loop (FMW), heat pump load-side supply- and return-water temperatures (temperature of heat pump supply [THPS] and temperature of heat pump return [THPR], respectively), building hot-water loop supply and return-water temperatures (temperature of building hot-water supply [TBHS] and temperature of building hot-water return [TBHR], respectively) and flow rate (FBH), and OA temperature (temperature of outdoor air [TOA]). A description of the collected data points is given in Table 1.

Analysis was conducted to characterize the performance of the demonstrated mine-water GSHP system; identify performance issues, if any; recommend potential improvements; and evaluate the energy savings and emission reduction benefits compared with a baseline scenario.

METHODS

The analysis was performed in two steps: first, the available measured data was analyzed to characterize the performance of the major components and the entire system; second, the full year energy consumption of both the demonstrated and a comparable baseline system was predicted to determine energy savings and environmental benefits from the GSHP system.

Measured Performance Characterization

Measurements of the data points indicated in Figure 2 were collected through a direct digital control (DDC) system at the facility. The grantee did not install any submeter for measuring the power consumption at the pump and the heat pump due to constraints in the budget. The DDC system polls the sensors once per second and provides 15 minute totals or averages of each sensor depending on the sensor type. Performance data from January 1 through July 31,2014 have been analyzed in this study. The measured data were analyzed to (1) assess the heat transfer performance of the mine-water loop, (2) determine the operational efficiency of the heat pump and the overall GSHP system, (3) assess the operation of the pump, and (4) identify faults/abnormalities in GSHP system operation and determine potential improvements to the GSHP system.

The coefficient of performance ([COPh.sub.sys]) and energy efficiency ratio ([EERc.sub.sys]) of the GSHP system were determined following the approach shown in Figure 3a. From the measured temperatures and flow rate in the mine-water loop, heat transfer rate at the mine-water loop (QGLA and QGLC) was calculated. The operating efficiency of the heat pump ([COP.sub.heqp] and [EER.sub.ceqp]), which does not include any pumping power, was determined as a function of the measured source-side and load-side leaving water temperatures (TMWR and THPS). Then heat pump power consumption (WHPA and WHPC) was calculated by dividing the heat transfer rate at the mine-water loop with the heat pump efficiency. Then, the output of the heat pump ([QHP.sub.h] and [QHP.sub.c]) was calculated with the heat transfer rate at the mine-water loop and the calculated heat pump power consumption. Finally, the overall GSHP system efficiencies were calculated as the ratio of the total output of the heat pump to the sum of the heat pump power consumption and the mine-water loop pumping power consumption.

Annual Energy Analysis

Annual energy analysis of the installed GSHP system was performed to (a) predict its full-year performance and (b) estimate the energy savings, operating cost savings, and emissions reduction benefits compared to a baseline system.

As discussed in the section "Analysis and Results," it was found that the mine water stayed warmer than the OA temperature in the summer and even warmer when mine water was used as a heat sink, suggesting that using the GSHP system to provide cooling would be less efficient than using the existing air-cooled chiller. Therefore, the annual energy analysis performed was based on the premise that the GSHP system would provide only heating.

Figure 3b shows the methodology to predict full-year GSHP system performance. First, the building heating loads (QBH) were predicted as a function of TOA based on a curve fit derived from the January through April measured data of the heating output of the building hot-water loop. Assuming an OA temperature reset schedule for the heat pump supply temperature (THPS, which is the heat pump load-side leaving water temperature) and a constant mine-water supply temperature (TMWS) as observed during the monitored time period, the heat pump heating capacity and heating coefficient of performance (COP) were determined by using the heating performance data provided in the product data catalog. The heat pump heat output (QHPA) was calculated on the basis of the assumption that the heat pump operates to satisfy the QBH until reaching its full capacity. Then the existing heating system will fill the gap between the building heating load and the heat pump capacity. The heat pump power consumption ([WHP.sub.h]) was calculated by using the heat pump heat output and heating COP. The mine-waterloop pump power consumption (WLP) was predicted from the pumping power ratio determined from the curve-fit for the daily mine-water loop pump energy use versus heat pump daily heat output (QHPH) derived from the January through April measured data. TMY3 weather data (WBT 2014) for Butte-Bert Mooney Airport, which is 6.5 miles (10.5 km) southeast of the site, is used for the annual energy analysis.

With the calculated heat pump power consumption (WHPH) and WLP, the total power consumption of the GSHP system was calculated and the overall system COP was determined.

ANALYSIS AND RESULTS

Key results from the data analysis are presented below. More detailed data analysis and results can be found in an accompanying technical report (Malhotra and Liu 2014).

Measured Performance Analysis

Mine-Water Loop Flow Rate and Temperature. The measured flow rates of the mine-water loop were 82-90 gpm (5.2-5.7 L/s) for most time during the data collection period (Figure 4). The measured mine-water loop supply and return temperatures, as well as the coincidental OA temperatures, are shown in Figure 4. As can be seen from this figure, the GSHP system operated intermittently in heating mode during January through April (indicated by lower return temperature than the supply temperature). The mine-water loop supply temperature was stable and was about 40[degrees]F (22[degrees]C) higher than the OA during most of the four-month heating period. On the other hand, in July when the GSHP system operated in cooling mode, the mine water was warmer than the OA.

The different behavior of the supply temperature is thought to be due to the convection heat transfer phenomenon in the mine water. When the GSHP system operated in heating mode, heat was extracted from the upper portion of the water body, making it cooler. Due to the existence of a density gradient in the shaft, colder water moved downward, and warmer water moved upward. It created convective heat transfer in the water body and maintained near constant leaving water temperature in the loop. However, when the system ran in the cooling mode, it rejected heat to the water and warmed up the upper portion of the water body. The warm water stayed at the top and became warmer until the conductive heat transfer through the wall of the mine balanced the heat rejected from the loop.

This suggests that at this site, using the mine-water GSHP system to provide cooling would be less efficient than using the existing air-cooled chiller. It is to be noted that the reduced heat transfer performance of mine water as heat sink in summer does not seem to be identified as a concern in other studies (Verhoeven et al. 2014; Wang et al. 2009; Banks et al. 2009). However, if mine water is used solely as a heat source or heat sink, issues of mine-water temperature degradation and unsustainable recovery of energy may eventually arise (Hall et al. 2011; Malolepszy 2003). The potential exists to use a small pump to move the warmer water to the bottom of the shaft to induce convective currents during the cooling season.

System Operation

The actual operation of the HVAC systems serving the building was investigated to understand system interaction, performance issues, and improvement opportunities. As shown in Figure 4, the mine-water loop pump operated intermittently during January through March, continuously in April, remained off for the entire months of May and June, and operated continuously in July. It was found that there were times when the heat pump was off but the mine-water loop pump was still running. A close inspection of the data revealed that when the mine-water flow rate was below 64 gpm (4.04 L/s), the heat pump was off. Even when the mine-water flow was 64 gpm (4.04 L/s) or above, the GSHP system ceased to operate when the building hot-water loop return temperature exceeded 124[degrees]F (51.1[degrees]C).

Based on the building control and operation documents, the building hot-water loop supply temperature was controlled according to an OA reset schedule: 135[degrees]F at-10[degrees]F (68.3[degrees]C at -23.3[degrees]C) OA temperature and 100[degrees]F at 60[degrees]F (48.9[degrees]C at 15.6[degrees]C) OA temperature, with a linear response in between. The scatter plot of measured supply and return temperatures of the building hot-water loop load side of the heat pump in Figure 5 confirmed that the OA reset schedule was effective for the most part. It also shows that the heat pump operated only when the return temperature from the building's hot-water loop, which is the return-water temperature to the heat pump (round dots shown in Figure 5), was below 124[degrees]F (51.1[degrees]C). This is due to the higher return-water temperature triggering the high discharge pressure protection and automatically shutting off the heat pump. This explains the intermittent GSHP system operation shown in Figure 5, even when the mine-water flow was 64 gpm (4.04 L/s) or above. The heat pump's heating operation was turned off when the OA temperature was higher than 60[degrees]F (15.6[degrees]C).

Table 2 summarizes the conditions of operation of the heat pump based on the observations from the measured data.

Further analysis of building hot-water loop supply and return temperature revealed that a significant amount of heating was delivered to the building even during the daytime in the summer (July). Figure 6 shows a scatter plot of building hot-water loop temperature difference and flow rate versus OA temperature. Note that the hot-water loop flow rate has a minimum (100 gpm [6.3 L/s]), due to the minimum speed limit (60% of the full speed) of the hot-water circulation pump. This minimum occurs when OA temperatures are above 65[degrees]F (18.3[degrees]C). Data points A correspond to building main heating loads during the winter and at night during the summer. Data points B correspond to the reheating loads during warmer weather when space cooling was needed in the building. Apparently, when the heat pump was providing nearly constant 50 ton (175 kW) cooling during daytime in the summer in addition to the existing 170 ton (595 kW) air-cooled chiller, the building hot-water loop had to be operated to provide reheat to variable-air-volume (VAV) terminal units in the building to maintain room temperatures at the setpoint. The minimum flow rate of 100 gpm (6.3 L/s) combined with high temperature difference corresponds to these periods, when the hot-water loop was providing reheat.

Figure 7 shows a weeklong snapshot of building hot-water loop temperature during heat pump operation in cooling mode. It shows the building hot-water loop supply and return temperatures, heat pump load-side supply and return-water temperatures, and OA temperature. Apparently, the heat pump was operating in cooling mode (indicated by the near 40[degrees]F [4.44[degrees]C] supply temperatures) during daytime and in heating mode (indicated by the above 120[degrees]F [48.9[degrees]C] supply temperatures) at night when the OA temperature dropped below 60[degrees]F (15.6[degrees]C). During daytime, when the heat pump was operating in cooling mode, the steam system had to provide reheat, which was indicated by the >100[degrees]F (37.8[degrees]C) supply temperature of the building hot-water loop and the coincidentally high temperature difference. Note that the campus steam system is run at low pressure during the summer to provide domestic hot water and to avoid pipe corrosion that would occur if steam were shut off for several months.

Heat Pump Efficiency

The efficiency of the heat pump, indicated by the COP for heating and the energy-efficiency rating (EER) for cooling, was determined from the equipment performance curve at varying source-side leaving water temperatures and load-side hot/chilled-water supply temperatures.

As shown in Figure 8, in the heating mode the measured source-side leaving water temperature exceeded the temperature range for which the manufacturer's performance data was available (i.e., 30[degrees]F-55[degrees]F [-1.11[degrees]C-12.8[degrees]C]). Therefore, the heat pump is expected to operate at higher efficiency in heating mode. However, for this analysis, conservative estimates of the COP were made by limiting the maximum source-side leaving water temperature to 55[degrees]F (12.8[degrees]C).

In cooling mode, the measured source-side leaving water temperature in some time periods during July exceeded the temperature range for which the manufacturer's performance data was available (75[degrees]F-105[degrees]F [23.9[degrees]C-40.6[degrees]C]) (see data points A in Figure 8). During these periods, the heat pump is expected to operate at lower efficiency. However, for this analysis, the heat pump cooling EER estimation was made by limiting the maximum source-side leaving water temperature to 100[degrees]F (37.8[degrees]C).

With these conservative adjustments, the calculated heating COP of the heat pump ranged between 3.2 and 4.7, and the cooling EER ranged between 12.7 and 19.5. Aggregating the heat delivered and power consumed by the heat pump, the average heating COP of the heat pump was 3.95 during January through April and the average cooling EER was 15.3 in July.

Pumping Power

The power draw of the 7.5 hp constant-speed pump is 4.476 kW, which is calculated based on an 80% average loading factor (i.e., 7.5 hp x 0.746 kW/hp x 80% loading). Assuming constant power draw when the pump was on (with higher than 64 gpm (4.04 L/s) flow rate in the mine-water loop), it is calculated that the mine-water loop pump consumed 4390 kWh (15,804 MJ) during January through April and 2263 kWh (8147 MJ) in July. Pumping performance is evaluated as the ratio of pump energy use relative to the total GSHP system energy use (referred to as pumping power fraction).

Figure 9 shows monthly aggregated power consumption of the heat pump and the pump, as well as the monthly average pumping power fraction. The pumping power contributed to about 12% of the total power consumption of the GSHP system during January through April (including power consumptions from both the circulation pump and the heat pump), after excluding the periods when the GSHP system was not operating because of operating condition limitations or otherwise. In July the pumping power fraction in the cooling mode operation was 13%.

GSHP System Efficiency

Figure 10 shows the overall efficiency of the GSHP system compared with efficiency of the heat pump in both the heating and cooling modes. As expected, the GSHP system efficiency is lower than the heat pump efficiency because of the pumping energy use. For the January through April measurement period, the heating COP of the GSHP system ranged between 3.0 and 4.3. Based on the measured data in July, the cooling EER of the GSHP system ranged between 11.8 and 15.6. Aggregating the heat delivered and power consumed by the heat pump and mine-water loop pump, the heating COP of the GSHP system was 3.49 during January through April and the cooling EER of the GSHP system was 12.8 in July. Accounting for the pumping power, the GSHP system has a heating COP of 3.5. Note that the GSHP system ran in cooling mode even when the TOA is below 60[degrees]F (15.6[degrees]C), which indicates that there must be some other control that switches between heating and cooling operation mode.

Recommended Heat Pump Operation

To achieve the maximum potential of the GSHP system, the system must be able to operate as much as possible within the manufacturer-specified range of the leaving source-side temperatures (30[degrees]F-55[degrees]F [-1.11[degrees]C-12.8[degrees]C]) and the leaving load-side temperatures (110[degrees]F135[degrees]F [43.3[degrees]C-57.2[degrees]C]). From the catalog data, the heat pump heating capacity at 55[degrees]F (12.8[degrees]C) leaving source-side water temperature ranges between 839.4 kBtu/h (246 kW) at 110[degrees]F (43.3[degrees]C) leaving hot-water temperature and 758.7 kBtu/h (222 kW) at 135[degrees]F (57.2[degrees]C) leaving hot-water temperature. Based on the assumption of 3 gpm/ton (1.08 L/s per 100 kW) for a 150 gpm (9.46 L/s) constant supply-water flow rate, the heat pump can provide a temperature differential of 10.1[degrees]F at 135[degrees]F (5.6[degrees]C at 57.2[degrees]C) and 11.2[degrees]F at 110[degrees]F (6.2[degrees]C at 43.3[degrees]C) leaving hot-water temperature. To provide this temperature differential, the heat pump would operate when the building return-water temperature is 125[degrees]F (51.7[degrees]C) or below or the OA temperature is 10[degrees]F (-12.2[degrees]C) or above. Below this temperature, the heat pump cannot operate due to the high return temperature from the building hot-water loop (according to the OA reset schedule discussed previously). The heat pump THPS thus derived is shown in Figure 11, which is used as a basis for the subsequent analysis.

Annual Energy Analysis

Predicted annual performance of GSHP system.

Assuming that the recommended improvements for the pumping control and heat pump operation are implemented, the annual contribution of the GSHP system for satisfying the building heating demands and the associated power consumptions were predicted following the procedure presented in Figure 3b. As shown in Figure 11, the heat pump would operate only when the OA temperature is at 10[degrees]F (-12.2[degrees]C) or above and the heat pump load-side supply water temperature is reset based on OA temperature. The existing hot-water system would be required only during hours when the OA temperature drops below 10[degrees]F (-12.2[degrees]C). With this operation, the GSHP system would have delivered 2882 MMBtu/year (844.6 MWh/year), which is 87.7% of the annual heating load (as shown in Table 3). The predicted heating COPs of the heat pump and the GSHP system for the full-year operation are 3.68 and 4.19, respectively. The pumping energy use is 12.2% of the total GSHP system energy use.

The predicted annual energy analysis results are plotted in Figure 12 along with the measured data during January through April 2014, and includes the hourly heat pump energy use (a), mine-water loop pump power fraction (b), heat pump heating COP (c), and GSHP system COP (d) against the heat delivered by the heat pump. Clearly, the predicted and the measured heat pump energy use match well because both use the same heat pump performance curves. The predicted mine-water loop pump power fraction is smaller than the measured data at lower heating loads due to the better pumping control, which runs the pump only when the heat pump is called on. The predicted heating COP of the heat pump is higher than the measured data, especially during milder weather with lower heating loads, which is a result of the additional OA reset schedule for the heat pump load-side supply-water temperature (shown in Figure 11). Combined with the higher operational efficiency of the heat pump and the smaller pumping power fraction, the predicted operational efficiency of the GSHP system is higher than the measured data at part-load conditions.

Comparison with the Baseline System. A comparison of the GSHP system was performed with the existing steam system as the baseline to determine the energy savings and environmental benefits. The existing system uses steam heating to provide hot water to the building, with the steam produced by a natural-gas-fired boiler. The heat is delivered to the building through a hot-water distribution system and terminal. For the baseline scenario, the existing system was assumed to provide the entire building's heating loads. The energy use of the baseline system was determined assuming 78% boiler efficiency. Energy use of the GSHP system scenario was determined as the sum of GSHP system energy use and boiler energy use, which provided the remainder of the heating loads. The source energy consumptions as well as the equivalent C[O.sub.2] emissions for the two systems are calculated using converting factors provided by Deru and Torcellini (2007) for natural gas and electricity in the region where the demonstration project is located. The energy cost for the two systems is calculated on the basis of a $9.63/MMBtu ($32.9/MWh) natural gas cost and a $0.08/kWh electricity cost obtained from the site. Compared with the baseline system, the GSHP system annually saves 2912 MMBtu (853 MWh; 69.1%) of site energy and 1769 MMBtu (518 MWh; 38.4%) of source energy and reduces 244,077 lb (122 tons; 39.3%) of C[O.sub.2] emissions, resulting in an annual savings of $17,227 (42.5% of the baseline).

DISCUSSION AND CONCLUSION

The measured performance data during January through July 2014 indicated that the GSHP system was operated intermittently from January through March and shut off in May and June.

Mine water available at the demonstration site is a stable heat source, and it can provide higher than 55[degrees]F (12.8[degrees]C) water to the heat pump. Based on the aggregated measured data from January through April, the heating COP of the heat pump and the GSHP system were 4.0 and 3.5, respectively. Further analysis of the predicted full-year performance shows that when fully utilized, the GSHP system would be able to satisfy 87.7% of the annual heating loads with an average heating COP of 4.2 and 3.7 for the heat pump and the overall system, which are 5%-6% higher than those measured during the four-month period. These increases are due to the additional OA reset control for the heat pump.

The pumping power annually accounts for 12% of the total GSHP system energy use. Excessive pumping occurred at times when heating demands were low due to continuous operation of the constant-speed pump in the mine-water loop.

Compared with the baseline system (i.e., the existing steam system using a natural gas boiler), the GSHP system achieved significant energy savings and C[O.sub.2] emission reduction. The GSHP system demonstrated 69% site energy savings, 38% source energy savings, and 39% C[O.sub.2] emission reduction compared with the natural-gas-fired boiler. The operating cost savings compared with the natural gas boiler system depend greatly on the natural gas price. For this analysis, a $9.63/MMBtu natural gas cost and a $0.08/kWh electricity cost were used (these were obtained from the site). The operating cost savings were $17,227 per year with these utility cost assumptions.

The installed cost ($750,000 for the mine loop HX, i.e., $15,000/ton) of the demonstrated GSHP system, which uses mine water as a heat source and heat sink, is higher than that of conventional GSHP systems (about $7000/ton on average in 2006 dollars, or $8200/ton in 2014 dollars) that use vertical-bore ground heat exchangers. As an experimental work to research the potential of using mine water as a potential heat source/sink, the project cost included other research-related costs, such as the effort for studying the convection mechanisms for geothermal heat exchangers in the vertical mine shaft (Thornton et al. 2013), which will not exist for nonresearch projects.

Lessons Learned and Recommendations for Further Improvements

To achieve its maximum potential, the GSHP system must be able to operate as much as possible within the manufacturer-specified range of operating conditions. Therefore, the supply and return temperatures of the existing heating system should be as low as possible. Also, its operating efficiency can be maximized by optimizing the leaving source-side and load-side water temperatures. The following measures can be used to achieve the following objectives:

* By optimizing the OA reset schedule for the building hot-water loop to allow lower supply- and return-water temperature and lower flow rate, the operation of GSHP system can be maximized even at peak heating load conditions while meeting the manufacturer-specified operating conditions.

* For new construction, a holistic system design approach is recommended to select and size the heat delivery equipment/terminal units that can work with relatively low hot-water temperature. A lower supply temperature would result in higher operating efficiency and maximum operation of the heat pump.

Control of heat pump operation should be improved to avoid a large amount of summertime reheat by the steam heating system caused by the heat pump operating in cooling mode. Pumping control could be refined to avoid wasting pumping energy when the heat pump is not in operation.

The transportation heat loss of the steam heating system was not accounted for in this study due to the lack of information. Should it have been accounted for, the energy savings from the GSHP system could be even bigger. As described previously, water temperatures in the closed-loop heat exchanger immersed in the mine water increased sharply during cooling operation due to the thermal stratification in the mine water. However, if convection in the mine water can be actively induced with a small pump, or mine water can be directly used within the heat pump, the mine water could serve as a better heat sink given the nearly constant 78[degrees]F (25[degrees]C) temperature.

ACKNOWLEDGMENTS

This work was sponsored by the U.S. Department of Energy, Building Technologies Office.

Notice of Copyright

This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http:// energy.gov/downloads/doe-public-access-plan).

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DISCUSSION

Jeffrey D. Spitler, Professor, Oklahoma State University, Stillwater, OK: The mine water temperature is quite a bit higher than the expected undisturbed ground temperature. What is the reason for this?

Xiaobing Liu: The mine water temperature was measured on site. According to a study conducted by Hagan (2015), water in the studied mine shafts is thermally well mixed, and the measured mine water temperature is 27[degrees]C at both 100 and 400 m below the surface. The observed geothermal gradient is 22.5[degrees]C/km at the site. Given that the depth of one of the mine shafts is 1 km and the mines are connected underground, it is believed that the 27[degrees]C mine water temperature is the result of the geothermal gradient and the mixing of water due to the convection driven by the temperature difference at the top and bottom of the shaft.

Hagan, T. 2015. Temperature and pressure sensing in three flooded underground mine workings in Butte, Montana, USA. Graduate theses and non-theses, Paper 21, Montana Tech, Butte, MT.

Xiaobing Liu, PhD

Member ASHRAE

Mini Malhotra, PhD

Associate Member ASHRAE

Adam Walburger

Member ASHRAE

Jack L. Skinner, PhD, PE

Donald M. Blackketter, PhD, PE

Xiaobing Liu and Mini Malhotra are research staff at Oak Ridge National Laboratory, Oak Ridge, TN. Adam Walburger is general manager at CDH Energy Corp., Cazenovia, NY. Jack L. Skinner is an assistant professor and Donald M. Blackketter is a professor at Montana Tech, Butte, MT.

Table 1. Description of the Collected Data Points

Data                Description                        Unit
Point

TMWS    Mine-water loop supply temperature    [degrees]F ([degrees]C)
TMWR    Mine-water loop return temperature    [degrees]F ([degrees]C)
FMW          Mine-water loop flow rate               gpm (L/s)
THPS     Heat pump loop supply temperature    [degrees]F ([degrees]C)
THPR     Heat pump loop return temperature    [degrees]F ([degrees]C)
TBHS          Building hot-water loop         [degrees]F ([degrees]C)
                supply temperature
TBHR          Building hot-water loop         [degrees]F ([degrees]C)
                return temperature
FBH      Building hot-water loop flow rate           gpm (L/s)
TOA           Ambient air temperature         [degrees]F ([degrees]C)

Table 2. Observed Operation of Heat Pump

Mine-Water Loop    Hot-Water Return     Hot-Water Return
Flow (FMW)        Temperature (TBHR)   Temperature (TBHR)
                    <124[degrees]F       >124[degrees]F
                   (51.1[degrees]C)     (51.1[degrees]C)

>64 gpm              Heat Pump on
  (4.04 L/s)        (heat pump was       Heat pump off:
  (pump on)       off occasionally)      steam HX alone
>0, <64 gpm         Heat pump off:
  (4.04 L/s)        steam HX alone
  (pump on)
0 (pump off)        Heat pump off:       Heat pump off:
                    steam HX alone       steam HX alone

Table 3. Summary of Predicted Full-Year Performance Data

                                     Unit       Full-Year
                                             Predicted (TMY3)

Building heating loads               MMBtu         3286
Cumulative heat output               MMBtu         2882
Cumulative heat pump energy use       kWh        201,335
Cumulative well pump energy use       kWh         28,038
Cumulative GSHP system energy use     kWh        229,374
% of building heating loads met        %           87.7
Average COP of heat pump              --           4.19
Average COP of GSHP system            --           3.68
Percentage of pumping energy use       %           12.2
COPYRIGHT 2016 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE)
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Author:Liu, Xiaobing; Skinner, Jack L.; Malhotra, Mini; Blackketter, Donald M.; Walburger, Adam
Publication:ASHRAE Transactions
Article Type:Report
Date:Jul 1, 2016
Words:6767
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