The authors propose a hybrid geothermal heat pump system that could cool buildings in summer and melt snow on the pedestrian sidewalks in winter, utilizing cold mine water and hot spring water. In the proposed system, mine water would be used as cold thermal energy storage, and the heat from the hot spring after its commercial use would be used to melt snow for a certain section of sidewalks. Neither of these sources is viable for direct use application of geothermal resources, however, they become contributing energy factors without producing any greenhouse gases. To assess the feasibility of the proposed system, a series of temperature measurements in the Edgar Mine (Colorado School of Mines' experimental mine) in Idaho Springs, Colorado, were first conducted, and heat/mass transfer analyses of geothermal hot spring water was carried out. The result of the temperature measurements proved that the temperature of Edgar Mine would be low enough to store cold groundwater for use in summer. The heat loss of the hot spring water during its transportation was also calculated, and the heat requirement for snow melt was compared with the heat available from the hot spring water. It was concluded that the heat supply in the proposed usage of hot spring water was insufficient to melt the snow for the entire area that was initially proposed. This feasibility study should serve as an example of “local consumption of locally available energy”. If communities start harnessing economically viable local energy in a responsible manner, there will be a foundation upon which to build a sustainable community.
Geothermal energy is a safe, 24/7 renewable energy source with a unique
ability for cascading usage, thus being well suited for use in the
development of a sustainable community. For example, Hachijojima is a
volcanic island located 300 km south of Tokyo, Japan with a population of
about 9500 according to Yamashita et al. (2000). The Hachijojima Geothermal
Power Station, operated by the Tokyo Electronic Power Company (TEPCO) since
1999, has the rated output of 3300 kW of electricity to meet about 30 %
of the electricity demand for this isolated island community. Before
commercial operation of the geothermal power plant, all electricity demand
was met by a diesel power station. In addition to the power
generation, the 43
Map of Idaho Springs, Colorado.
Although there are no geothermal power stations in Colorado, USA, the state is blessed with low-enthalpy geothermal resources. Local residents and tourists enjoy natural hot springs in many places. The Geo-Heat Center Quarterly Bulletin has described six Colorado hot springs and direct use applications for those springs. One of the side effects of Colorado's booming mining economy during the middle of the 19th century was that there were many mines in the state that were left abandoned when the mining boom ceased.
Abandoned mines are usually environmental and safety liabilities for
communities in which they are located, but their potential use as thermal
storage resources should not be overlooked. Pingjia and Ning (2011) studied
three different usages of abandoned mines, and one of the usages identified
in their study is the “Thermal Accumulator”. In this usage, two
underground mines, a cold mine and a hot mine, would be identified and
utilized. Cold water stored in the cold mine would be used in summer, and
warm water stored in the hot mine would be used in winter to improve
geothermal heat pump efficiency. In a different study, Rodriguez and Diaz (2009)
considered a deep underground mine in Spain to be used as a
geothermal heat exchanger. According to their measurement, the temperature
of the rock mass in the underground mine was a constant 27
The city of Idaho Springs, Colorado, USA, is located about 50 km west of Denver in the foothills of the Rockies. The town was founded in 1859 by mining prospectors and flourished as a mining community throughout the 1860s. Today, the town has a population of about 1900 and attracts tourists to its historic downtown, hot spring, and the experimental Edgar Mine located just north of the town. Figure 1 shows the map of the city of Idaho Springs and the locations of the historic downtown, the hot spring, and the Edgar Mine.
The proposed heating/cooling geothermal heat pump system for the city of Idaho Springs can be visualized in Fig. 2. It was assumed that the rock mass temperature in the Edgar Mine would be relatively low since the mine was located at a shallow depth. Therefore, the authors envisioned that the collected cold groundwater would be stored in a closed section of the Edgar Mine in winter and used to cool down the condensers of heat pumps in summer as shown in Fig. 3a. In this way, the efficiency of heat pumps would be improved. In winter, geothermal hot spring water used for commercial bathing would be transported to the historic downtown and used to melt the snow on the pedestrian sidewalks. Furthermore, any residual heat from the hot spring water after snow melting could be used to heat the heat pump evaporators, which would improve the efficiency of the heat pumps (Fig. 3b).
Proposed system for Idaho Springs.
As reviewed in the previous section, the use of a mine to improve heat pump efficiency is not a new idea. For example, Shiba (2008) also reported a case study in which hot spring water consumed in a public bathing facility was re-used to improve heat pump efficiency of buildings in Japan. However, to our best knowledge, we know of no case study in which a mine and hot spring are jointly utilized to assist in improving the efficiency of a geothermal heat pump system. The close proximity of a hot spring and available mine also makes the location of Idaho Springs and its potential distribution of a geothermal resource unique.
Interaction between proposed system, heat pump, and building.
We also point out that there are acceptable flow and temperature range with
heat pump source water. The extended range water–water heat pumps prefer a
source and load flow rate of 2.5 to 3.0 gpm per nominal ton of capacity,
i.e., 0.16 to 0.19 L s
The Edgar Mine produced high-grade silver, gold, lead and copper in the mid
19th century. Colorado School of Mines (CSM) acquired the Edgar Mine in
1921 when a bankrupt mining company agreed to lease it to the school, and
CSM has been using the mine for education and research ever since. As an
example of its use by the school, junior students in the Mining Engineering
Department take a course entitled “Mining Engineering Laboratory” at the
Edgar Mine where they receive practical training in operating jackleg
drills, jumbo drills, load–haul–dump machines, etc. In other classes,
students gain hands-on experience in underground mine surveying, geological
mapping, mine ventilation field studies, mine safety, and so on. Photo 1
shows the entrance of the Edgar Mine (Miami Tunnel), and Photo 2 shows a
classroom inside the mine. Research is conducted at the Edgar Mine by
numerous academic, government, and industry groups including the CSM Mining
Engineering Department, the National Institute for Occupational Safety and
Health (NIOSH), the US Army, the US Department of Energy and others.
Research topics cover tunnel detection, blasting, rock mechanics,
development of a model circulation system for geothermal study, and
development of new mining equipment and methods. For more information, the
CSM website is available using the following link:
Ambient temperature, precipitation, and snowfall in Idaho Springs.
Map of Edgar Mine and rock surface temperature at 24 locations.
Relationship between temperature and distance from the entrance.
In order to assess the thermal capacity of the Edgar Mine, the temperature field inside the Mine was mapped. Temperature measurements were carried out on three different dates; 17 September, 24 October, and 25 November of 2013. Figure 4 shows the data for temperature, precipitation, and snowfall in Idaho Springs in 2013. The data were obtained from the website of AccuWeather.com. The ambient temperature decreased significantly during the measurement period. Measurements taken include the surface temperature of the rock mass, ambient temperature, and humidity at the 24 locations shown in Fig. 5. There are two main tunnels in the Edgar Mine. The eastern tunnel is called the Miami Tunnel, and the western tunnel the Army Tunnel. The height and width of the Miami Tunnel are about 2 and 2 m, and those of the Army Tunnel are about 4 and 5 m. An infrared thermometer (Fluke, Model: 62Max) was used to measure the rock surface temperature as shown in Photo 3. The temperature of groundwater accumulated at location 21 was also measured. The area between the entrance of the Army Tunnel (Location 24) and the location 19 was wet during this period.
The measured temperatures of the rock mass surface at the 24 locations are
shown in Fig. 5. The temperature measured near the entrance of the two
tunnels decreased during the three measurements due to the influence of the
ambient temperature. On the other hand, the temperature inside the mine was
stable. The highest surface temperature was always measured at location 9
(U.S. Geological Survey (USGS) classroom), and it was about 12.5
Compared to the surface temperature of 27
Figure 6a and b show how the measured rock surface and the ambient temperatures change with the distance from the entrance. More specifically, the temperatures measured at locations 1, 2, 3, 4, 5, and 6 are shown in Fig. 6a, and the temperatures measured at locations 18, 19, 21, 23, and 24 are shown in Fig. 6b. Comparing the surface temperatures for locations near the entrance to each of the two tunnels, it is found that the surface temperature increases with the increasing distance from the entrance in the Miami Tunnel (Fig. 6a), and the surface temperatures at locations 4, 5, and 6 show no significant difference between the three measurement dates even though the ambient temperature decreases. Figure 6b, on the other hand, indicates that the surface temperature does not increase as the distance becomes greater in the Army Tunnel. Additionally, the surface temperatures shown in Fig. 6b are significantly different for each of the three measurements.
In order to understand the difference in the temperature profiles between the Miami Tunnel and the Army Tunnel, the relationship between the cover that is defined as the difference between the elevation of ground surface and that of the tunnel and the distance from the entrance is shown in Fig. 7. It can be seen that the cover above the Army Tunnel does not increase as much as that for the Miami Tunnel. The heat is transferred through the rock mass by conduction since no significant wind blows inside the underground mine. Therefore, as the cover becomes greater, the surface temperature of the rock mass is less influenced by the ambient temperature outside the Edgar Mine. The rock surface temperatures at locations 4, 5, and 6, with the cover greater than 150 m, is considered to be independent of the ambient temperature outside the mine. Figure 7 also shows the different topography above the Miami Tunnel and the Army Tunnel, which should explain why the Miami Tunnel is dry and the Army Tunnel is wetter. The thin and relatively flat cover above the Army Tunnel provide more opportunity for the surface water to permeate through to the tunnel.
In conclusion, it is found that the surface temperature of the rock mass in the Edgar Mine is relatively low due to its shallow depth. Therefore, the mine would be suitable for thermal energy storage in which cold groundwater would be stored in winter and used in summer as proposed in Fig. 2. Our recommendation is to utilize the groundwater that is naturally flowing in the mine in a designated storage area.
In the proposed system, hot spring water will be transported from the hot spring to the east end of historic downtown area through a pipe buried in the ground. In this section, the temperature loss of the transported hot spring water is estimated.
In order to understand the heat transfer between the flowing hot spring
water and the ground, the ground temperature
Relationship between cover and distance from the entrance.
Figure 8 shows the temperature profile in the ground at various depths and
indicates that the ground temperature becomes more or less a constant value
of 6.7
Figure 9 shows the schematic representation of the hot spring water flowing
to the historic downtown. According to Repplier et al. (1982), the flow rate
of the geothermal well used by the hot spring is 136 L min
The Reynolds number of the internal flow through a pipe,
Ground temperature as a function of time and depth.
Schematic representation of the geothermal hot fluid flowing from hot spring to historic downtown.
Relationship between mean temperature of geothermal hot fluid and travel distance.
Convection heat transfer coefficient,
Total thermal resistance per unit length,
Relationship between outlet temperature of geothermal hot fluid and pipe thickness.
Schematic representation of the geothermal hot fluid flowing from hot spring to historic downtown.
Schematic representation of geothermal hot fluid flowing from hot spring to historic downtown.
Figure 10 shows the relationship between the mean temperature of the
geothermal hot spring water and its traveled distance. It is found that the
temperature would decrease from 35 to 28.6
The exiting temperature 28.6
Figure 12 shows the design of the proposed snow melting system for the sidewalks of the historic downtown. The hot water transported from the hot spring source flows through the pipe system in the north and south pedestrian sidewalks. The depth, the spacing, and the size of buried pipes are appropriately adapted after having reviewed two previous case studies in which similar snow melting systems were installed. One case study covers the system in Klamath Falls, Oregon, reported by Lund (1999) and the other covers the system in Sapporo, Japan, reported by Sato and Sekioka (1979).
In order to calculate the heat supply from the hot spring water to the
pedestrian sidewalks, the temperature decrease of the flow through the pipes
buried in the sidewalks was calculated in the same way as the calculation of
the temperature loss in Sect. 4. However, the buried depth of the pipes
shown in Fig. 12 is only 8 cm, while that of the pipe in the previous
section was assumed to be 2 m. Therefore, the geothermal hot water would
significantly be influenced by cold ambient temperature. Thus, it is assumed
that the flow is cooled down by the constant temperature,
The outlet temperature
Chapman and Katunich (1956) estimated the required total heat flux for snow
melting,
Sensible heat flux,
Latent heat flux,
Convective and radiative heat flux from a snow-free surface,
Evaporation heat flux,
As a result, the required total heat flux for snow melting is 229.1, 324.0,
and 418.9 W m
Comparing the heat supply from the geothermal hot fluid (124.0 kW) with the
required total heat shown above, it is found that the heat supply would be
insufficient to melt snow even when
In this study, the authors proposed a hybrid geothermal heat pump system that is coupled with mine water and hot spring water. We mapped the temperature profile in the Edgar Mine, assessed its thermal capacity, and analyzed the heat/mass transfer of the geothermal hot spring water.
The temperature measurements showed that the temperature of rock surface was
approximately 12
The heat/mass transfer analyses showed that the temperature of the
geothermal hot water decreased from 35 to 28.6
The energy balance analyses showed that the proposed system would not melt
snow-covered pedestrian sidewalks effectively. In order to satisfy the heat
requirement with
The authors would like to thank Terry Proffer for his critical review of the manuscript. Edited by: R. Schulz Reviewed by: T. Proffer and S. Rumohr