Combining HVAC and GSHP for De-icing and Snow-melting

Key words: GSHP, ground source, geothermal, heat pump, de-icing, snow-melting, BTES, ATES, hydronic heated pavement, HHP

At outdoor temperatures around zero centigrades, pedestrian slipping accidents increase, often leading to painful and disabling injuries, sometimes even fatal, especially among elderly people. Thermal de-icing of paved surfaces using a building’s ground source heat pump system is a cost-effective and eco-friendly solution to reduce such slipping accidents.

Introduction

Slip accidents are common and often so severe that the victim needs medical attention. In the worst cases these accidents may be severely disabling or even fatal. The accidents are usually caused by icy conditions, often occurring when the outdoor temperature hovers around zero degrees Celsius. Electric or hydronic ground heating systems are sometimes installed to avoid such incidents - a market that has increased with approximately 6% annually in the past 20 years. Applying ground source heat pumps (GSHP) is a cost-effective as well as eco-friendly solution for Hydronic Heated Pavement (HHP) systems. This can be applied for all sorts of GSHP systems in practically any scale, also systems that supply both heating and cooling to the building, The integration with HHP systems maximizes the ground source utilization, and saves energy compared to traditionally heated HHP at a significantly reduced energy cost. Furthermore, manual and chemical snow and ice removal methods often come with significant costs and environmental impact.

This article is based on the work within the international collaboration project IEA HPT-Task 38 that explores the usage of geothermal energy for snow melting and de-icing. For details, see Reference.

The HHP system in overview

An HHP system consists of three main components: a heat source, a piping system beneath the heated surface, and a control and monitoring system. The piping system is constructed with a top layer of asphalt, concrete or paving stones, and a network of plastic pipes at a depth of 50–100 mm with a c/c distance of 150–250 mm, see Figure 1.

Figure 1. Schematic of a hydronic heated pavement (HHP) for de-icing and snow-melting.

Power requirements for HHP de-icing and snow-melting vary depending on snowfall intensity, wind speed, and the chosen operating strategy. Typically, the required melting capacity is defined by the rate of snowfall that can be continuously melted. For example, an intensity of 2 mm/h (as melted water), requires a capacity of 200–250 W/m². For preheating the HHP surface ahead of a snow fall or sub-zero temperatures, the capacity demand is lower, usually in the range of 50–100 W/m². Additionally, if you want a wet surface to dry, this is usually achieved with a capacity of 100–150 W/m². The maximum supply temperature is commonly around 25–30°C. With a modern control system, both the supply temperature and flow rate are automatically adjusted for preheating, different snowfall intensities, and drying (evaporation). Typical figures for colder climates are shown in Table 1.

Table 1. Surface temperature, capacity demand, and supply temperature (measured at a pier for Stockholm Arlanda airport 2006-2007).

It is primarily the amount of snow that needs melting along with the strategy for keeping surfaces warm and dry, that determines the required heat output. The local climate condition defines how much energy is used. Experience from the Scandinavian countries indicate an annual requirement in the range of 150–250 kWh/m² of which preheating represents about 60–70% and snow melting about 10%.

HHP integrated to GSHP systems for heating only

Unlike air source heat pumps, GSHPs for heating only, utilize the relatively constant temperature of the soil and rock, which in countries with snowy winters, typically ranges between 3-10 degrees, reflecting the mean temperature in the air. For vertical Borehole Heat Exchangers (BHE), 1.5 up to 2.5 degrees per 100 m depth should be added due to the influence of the geothermal gradient. For horizontal pipes, often placed one meter below the ground surface, a significant part of the extracted heat origins from phase change as water in the soil freezes to ice.

Since the HHP system normally is active only in weather conditions around the freezing point and some 5 degrees below, a GSHP system for a building with an integrated HHP does not require any additional heat pump power. However, an enlargement of the heat source may be needed, to compensate for a larger extraction of heat. A schematic is shown in Figure 2.

Figure 2. Simplified flow chart for combining GS heating with a HPP system.

Another solution, probably less expensive, is to use the HHP system as a solar collector in the summer. This solution is only possible for systems using Borehole Heat Exchangers (BHE) as the ground source. In horizontal pipe systems the stored heat will dissipate before the heating season starts. A schematic of a system combining the BHE ground source with HHP solar collection is shown in Figure 3.

Figure 3. Simplified flow chart for combining heating and HPP system with BHE ground source using the HHP system for solar heat collection.

As shown in the Figure 3 it takes an additional loop with a heat exchanger to realize. By definition, the system then becomes a UTES system (Underground Thermal Energy Storage), see below.

According to the results from Task 38, the capacity of HHP systems for harvesting solar energy ranges between 300–400 kWh/m² per year, which is roughly twice as much as is consumed during winter operation.

The UTES systems are used for heating and cooling of large-scale applications such as airports, university campuses or hospitals, etc. The two most common types are: Borehole Thermal Energy Storage (BTES), and Aquifer Thermal Energy Storge (ATES). In Scandinavia BTES is the dominating system type, while the Netherlands is the main user of ATES. The two systems are illustrated in Figure 4.

Figure 4. The BTES and ATES concepts for heating and cooling of commercial and institutional buildings.

The main difference between the techniques is that ATES has a warm and a cold side while BTES relies on heating and cooling a rock mass penetrated by a large number of closely spaced BHEs. In ATES systems the energy from the aquifer is directly transferred to the HVAC system by a plate heat exchanger, whereas BTES are using the same fluid in all brine circuits, usually a mixture of water and ethanol or glycol. An important distinction is that ATES can provide temperatures that will increase the COP of the heat pump and provide 100% free cooling to the building. This makes ATES more efficient compared to BTES. On the other hand, BTES offers other advantages, such as high operational reliability, virtually no maintenance, and an exceptionally long service life. They can also be installed in almost any geological setting, while ATES is limited to aquifers that are not used for drinking water supply.

HHP integrated to UTES systems

HVAC design using BTES

A schematic of an HVAC design using BTES is shown in Figure 5. As can be seen there are three sources for storage in the BTES system (1) waste heat from the cooling system, (2) waste heat from the condenser side of the heat pump when used for cooling, and (3) solar heat from the HHP system.

Figure 5. Proposed HVAC design for combined heating, cooling and snow melting system with BTES applications.

The heat sources are competing for the storage space in the BTES in the summer. In principle, waste heat from the cooling system should be the primary source and the condenser heat the secondary while heat from the HHP installation becomes the third. Therefore, solar heat can only be stored temporarily when the return from the cooling system is a few degrees lower than the temperature provided by the HHP system. This would be a challenge for the control system to handle and perhaps some of the wate heat from the condenser must be disposed to the air (not shown in the Figure 5). Still, the system is designed to handle and interact between all three sources.

For parallel connection of boreholes in the BTES system, two manifold pipes are used. These can be placed indoors, but commonly outdoor field manifold chambers beneath surface are used. The pipes to or from a borehole are equipped with a regulating valve (see Figure 5) for optimising the flow rate, which under normal conditions is around 0.50–0.60 l/s (DN40 mm).

The sizing of the main BTES circulation pump is based on the calculated total hydraulic pressure drop in the boreholes, heat exchangers, and the evaporator, along with the maximum flow rate. The pump is usually fitted with frequency control to adjust the flow rate to different operating conditions.

In Figure 5 the types of fluid for each circuit, are shown separated by plate heat exchangers. The brine is commonly a mixture of water and ethanol (28 %) that reduces the freezing point to approximately -11^oC. The brine in the HHP system is commonly glycol with a chosen freezing point based on the local climate conditions.

The Figure 5 displays control valves used to regulate flow for various operational modes. Several temperature meters, not shown in the figure, are part of the control system and monitor temperatures during different operating conditions. The temperature meters play an important role for managing heat storage during summer operation.

System design using ATES

ATES systems are less affected by mixing different heat sources than BTES. Still, waste heat from cooling is the main source, with solar heat from the HHP system as the secondary source. Cold supply from the aquifer's cold side remains quite constant regardless of the temperature on the warm side. Therefore, the cold side will cover practically the entire cooling demand. Still, the heat pump may produce peak cold and for that reason the system is designed to store condenser heat as well. Figure 6 illustrates how an HHP system typically can be integrated into an ATES system.

Figure 6. Proposed HVAC design for combined heating and cooling and a HHP system for ATES applications.

The advantage of having both warm and cold wells means a significantly greater number of control valves, which function is to always ensure counterflow in the heat exchanger whenever there is reverse flow from the aquifer (see the detailed drawing in the Figure 6).

The groundwater loop operates using submersible pumps located below the groundwater level within the production wells. The return water is injected through an injection pipe equipped with a control valve, see detailed drawing. Check valves in the pumps prevent short-circuiting. The groundwater loop is sealed and pressurized, which limits air exposure that could otherwise lead to corrosion and precipitation of wells and the groundwater heat exchanger.

The submersible pumps are sized with respect to the maximum flow rate, the hydraulic flow resistance in the aquifer and hydraulic pressure drop in pipes and heat exchanger. They are commonly also frequency controlled to adjust the flow rate to momentary need.

The main reason to have a heat exchanger between the groundwater circuit and the evaporator side of the heat pump is to prevent corrosion or precipitation of minerals, such as iron hydroxide, in the evaporator and heat exchangers.

Operating costs and profitability

In Task 38, various systems with GSHPs were studied using a number of examples regarding saving potential and profitability. However, for ATES and BTES, these were sized to be used solely for HHP applications, which means the results are not directly applicable to UTES systems designed for both heating, cooling, and HHP heating combined.

Task 38 also demonstrated that it is not possible to generalize energy prices, as these vary between countries and even regionally within many countries. Therefore, as a basis for this article, it has been assumed that electricity is used both for the GSHP system and for the alternative HHP heat source as well. The price of electricity has been set at 200 Euro/MWh (winter). This is a rough simplification but necessary in order to illustrate the difference in conventional heating compared to ground source heating of HHP systems.

The additional HVAC investment for connection and heating of the HHP system is an estimation based on practical experience, which also applies to the system’s SPF1 (Seasonal Performance Factor 1). This factor summarizes the electricity used for the heat pump and for heat source circulation pumps. Table 2 presents calculated cases for the various ground source alternatives, where the conventional alternative is an electric boiler that covers 100% of the heat to the HHP system.

Table 2. Calculated examples of profitability for the different GS systems in relation to the estimated additional investment in the HVAC system.

1) System with HHP solar collection

The table should only be regarded as a template for values. In reality, profitability is highly dependent on the actual price of electricity and the alternative energy source used for heating the HHP system. It should be noted that SPF is not a linear factor. The difference in energy savings between each SPF step therefore becomes increasingly less significant the higher on the scale one is. That is why the difference in using ATES (SPF 8) or BTES (SPF 5) is not very pronounced when it comes to savings and profitability, even though the additional investment cost is practically the same.

Environmental Benefits

One compelling argument for using GSHPs and UTES as heat source for HHPs for ice and snow melting lies in the limited environmental impact. Traditional HHP systems rely heavily on electric cables or fossil fuels, and chemical ice and snow removal methods require salt or chemical agents, all of which carry ecological consequences. The advantages of ground source heat for HHP are as follows:

·         Reduced Greenhouse Gas Emissions: Geothermal alternatives dramatically cut carbon emissions compared to any traditional heat sources.

·         No Chemical Runoff: Geothermal systems prevent from environmental affects by salt and chemicals and preserve the local environment.

·         Lower Noise and Air Pollution: Geothermal systems work quietly and do not produce on-site combustion emissions.

·         Less reliance on mechanical snow cleaning: Using geothermal systems mean less reliance on labour- and fuel-intensive winter maintenance practices.

·         Resource Efficiency: Integration of HHP with geothermal heating systems maximizes the use of installed infrastructure, resulting in a smaller environmental footprint.

These environmental benefits should all be of great interest to consider for private building owners as well as public stakeholders. This will strengthen the market position for combined GSHP-and HHP systems.

Conclusions

The combined use of ground source heat pumps and underground storage systems for de-icing and snow melting systems represents a forward-thinking solution for winter-time safety and sustainability.

With superior energy efficiency, significant environmental benefits, and favourable economic returns using ground source technologies, this is a promising way to decrease the number of slip accidents and manage infrastructure in the winter season.

As governments, businesses, and homeowners seek to reduce their carbon footprint and operating costs, ground source technologies for snow melting and de-icing systems offer a practical path toward a cleaner, safer, and more resilient future.

Acknowledgement

The funding contribution by the Swedish Energy Agency, Grant 51491-1, for the Swedish participation in IEA ES TCP TASK 38 is gratefully acknowledged.

Reference

IEA Energy Storage TCP Task 38 - “Ground Source De-Icing and Snow Melting Systems for Infrastructure” Report WP1, Delivery D1.2 “Market Potential for Ground Source Energy Applications”. This report forms the basis for the content in this article and can be downloaded from the Swedish Geothermal Energy Centre’s website: https://geoenergicentrum.se/geoenergi-2/iea-es-task-38-isfri-infrastruktur-med-geoenergi/