Ground Source Heating of Soccer Fields: Systems and Market Potential in Cold Climates

Introduction
Soccer is one of the largest sports worldwide. It is also a sport that is played all year round even in cold climates. In this type of climate, artificial turf is often used, mainly to prolong the active season and to cut the maintenance costs compared to natural grass fields.

In Europe there were some 90,000 artificial turf fields in 2020. There are no official statistics on how many of these fields are heated. In Scandinavia, with approximately 3,500 artificial turf fields, around 10% of them are heated, mainly with heat from district heating. Elsewhere in Europe electric cables and gas boilers are the dominant heat sources. The increasing energy cost in recent years has become a challenge for both the field owners and the soccer clubs. In order to significantly decrease energy costs, ground source heating is an attractive alternative. With years of experience from the space heating sector, ground source heating systems can be commercially competitive for heating of soccer fields. Ground source systems will also greatly reduce the CO₂ emissions compared with electric cables and fossil fuel boilers.

Artificial turf structure
The turf structure is principally the same regardless of the field being heated or not, and the national soccer associations provide guidelines on how to construct and maintain the fields. In a modern design, the heating coils are placed in a shock absorbing pad as shown in Figure 1.

From top to bottom, the layers consist of the artificial turf with elastic granules added; a thin layer of fine sand; a permeable shock pad; and a sand or gravel layer with embedded drainage piping. The field surface has a slope of approximately 1% from the middle line towards the sides to promote water runoff. The heating tubing is commonly made of PEX with a diameter of 20-25 mm (3/4 to 1 inch). The distance between the tubes is typically 200 mm (8 inches).

The critical component for heat conduction from the fluid to the surface is the granules in the plastic mat. The granules commonly have a low thermal conductivity that requires a relatively high supply temperature to the tubing. This also makes harvesting of solar energy more difficult. On the other hand, the shock pad, which also has a low thermal conductivity, will act as an insulating layer beneath the heating tubing.

Load and energy requirements
The common strategy is to keep the turf free of frost down to an ambient temperature of -20^°C (-4^°F). Hence, the heating demand is primarily determined by the outdoor temperature, but also by the wind speed. This design allows the melting of light snowfall at temperatures around and just below the freezing point, while more heavy snowfall must be handled mechanically.

Actual measured values are rarely reported but there are two examples from Scandinavia. In Uppsala, Sweden, a power of some 50 W/m² (15.6 Btuh/ft²) was needed, and for a pitch in Helsinki, Finland, a demand of 70 W/m² (21.8 Btuh/ft²) was measured. The annual heating demand in these two cases was found to be approximately 800 MWh (120 kWh/m² [11.1 kWh/ft²]) and 700 MWh (90 kWh/m² [8.4 kWh/ft²]) respectively. The supply temperature to the heating coils at -20^oC (-4^°F) was about +30^oC (+86^°F) in both these cases.

In general the annual energy demand is related to the number of days with frost temperatures. These vary but can be up to 250 days in Northen Scandinavia. Based on a survey that covered 15 full sized arenas (8,000 m² [86,111 ft²]) located all over Sweden, the annual heat demand was measured to a mean value of approximately 1,000 MWh (125 kWh/m² [11.6 kWh/ft²]).

PROMISING GROUND SOURCE systems
The groundwater temperature may in some cases be high enough to be directly used to keep a soccer field above the freezing point over most of a winter. However, a more effective solution is to use an Aquifer Thermal Energy Storage (ATES) in which the soccer field is used as a solar collector in summer and heat is stored until the winter (Figure 2).

The main advantage of using ATES systems is the separation of the aquifer into a “warm” and a “cold” side which makes it possible to store and produce a relatively high temperature in the winter season. If a heat pump must be used for peak demands, a favorable COP can be expected. Due to a large fraction of direct heating, the ATES system will often have a seasonal performance factor (SPF) in the range of 8-10.

The disadvantage with ATES is that suitable aquifers are rarely found in the immediate vicinity of the sports facility. Furthermore, there may be regulations that restrict the use of groundwater, and the groundwater chemistry may be unfavorable when it comes to clogging of wells. Altogether, this may restrict the market potential for using ATES.

A storage system that can be applied in almost any geological setting is Borehole Thermal Energy Storage (BTES). It consists of several vertical closed-loop boreholes (Figure 3). Single U-pipes are installed in the boreholes and connected in parallel. In Scandinavia, the boreholes are normally filled with groundwater as shown in the figure. However, borehole sealing with grout is mandatory in most other European countries.

Due to the high thermal resistance of the U-pipes, the supply temperature from BTES will be considerably lower compared to ATES. Therefore, a heat pump is necessary to meet the supply temperature criteria. With half of the heat supply as direct heating and the other half with the heat pump, the SPF will be in the range of 6-8.

An economic advantage of BTES systems is that the boreholes have a long lifetime (>50 years) that will cut the annual capital cost. Another advantage is that the borehole system is practically free of maintenance costs.

Hybrid BTES systems
In these systems surplus heat, such as condenser heat from an ice hockey arena, is stored in the ground instead of being disposed to the atmosphere. Chilling the condenser with the brine from a BTES system gives a win/win situation since BTES cooling is more efficient than using a dry condenser cooler at high outdoor temperatures, Figure 4.

Condenser heat from an ice rink cooling machine will primarily be used directly for heating the soccer field whenever the cooling machine is running to maintain the ice. However, in autumn, spring, and early summer when there is no heating demand for the soccer field, the surplus heat is stored in the BTES system at a typical temperature of +35^oC (95^oF). This heat, together with captured solar heat from the soccer field, is then recovered in the winter when the cooling machine is not in full operation.

In Sweden, there are four examples of hybrid systems. One of these is Backavallen in the city of Katrineholm, middle Sweden. Most of the stored heat is obtained in the autumn when the first ice is made. In winter and early spring, the cooling machines are run at low run-time fractions to maintain the ice. Over a full season, the machines emit about 1700 MWh of condenser heat, which is recycled in Backavallen's energy system. Some 1300 MWh of this heat is stored in the BTES which also stores around 400 MWh of solar heat from the artificial turf.

Economics
To be economically feasible the additional investment cost for installing a ground source system should be paid off in a reasonable time by the savings it provides. The payback equation may be simple, but the additional investment cost is highly dependent on local geological conditions, the drilling technology, and the cost for electricity and other heat sources. These factors vary from country to country and for that reason the economics of a ground source heating system is site specific.

As an example from Sweden, the additional investment for the BTES system at Backavallen was approximately SEK 10 million SEK (~1 million USD). With an energy saving value of 1 million SEK (US$100,000), the system was estimated to be paid back over approximately 10 years. For the recently installed system in Sipoo in Finland, the investment in the BTES system was €660,000 (US$660,000) and the annual energy saving to €90,000 (US$90,000) giving a straight payback time of 7.5 years.

As a bonus the ground source system will emit considerably less environmentally harmful gases compared to conventional heating. However, this advantage cannot currently be priced other than a “goodwill”.

CONCLUDING remarks
In Scandinavia approximately 10 % of the existing artificial turf soccer fields are heated, mainly by district heating. The proportion of heated artificial turf soccer fields in other parts of the world is not known, but common heat sources are electric cables and boilers. The large number of artificial turf soccer fields in Europe alone, indicates a large market potential for ground source heating. These can be directly applied for new construction, but also as a retrofit of existing conventional heating systems.

If designed for 50-75 W/m² (15.6-23.8 Btuh/ft²), the ground source heating systems are likely to keep a soccer field free of frost down to -20^oC (-4^oF). The supply temperature to the tubing at an outdoor temperature of -20^oC (-4^oF) will be about +30^oC (+86^oF). The annual heat demand varies with the number of days with a temperature at or below 0 ^oC (32^oF). A rough mean value would be around 100 days with an annual heat consumption of some 120 kWh/m² (11.1 kWh/ft²).

There are several advantages of using ground source systems for this application. The energy cost savings can be expected to be 80-90 %, making the systems highly cost effective from an operational point of view. In addition, the maintenance cost for closed loop systems is practically zero. Furthermore, ground source heat can be found in any geological setting and is free of charge, has no taxes, is totally renewable, and is less sensitive to future energy cost increases. These advantages make ground source heating an excellent alternative in the soccer field heating market.

The main disadvantage is the high initial investment cost for wells and/or closed loop boreholes. However, these system components can be paid off over a long period, often greater than 50 years. The long depreciation period will minimize the annual capital cost. Hence, a high initial investment cost should not be regarded as a serious market obstacle.

ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support for their work from the Swedish Energy Agency, Grant 51491-1, and from their employers.

REFERENCES
This article is a shorter version of the paper with the same title and authors published in the proceedings of IGSHPA Research Conference Montréal, May 28-30, 2024. (https://doi.org/10.22488/okstate.24.000025 ). Minor alterations have been made based on findings within the ongoing project IEA-ES-TASK 38. Ground Source De-Icing and Snow Melting Systems for Infrastructure.