Achieving net zero with geothermal systems
The Dept. of Defense Education Activity (DoDEA) has established aggressive energy goals for new schools, including several “net zero” facilities in 2011. According to the Dept. of Defense (see press release 319-11), a net zero energy building generates as much energy as it uses. This is based upon annual total energy use and generation, which allows on-site energy sources to be sized for average loads instead of peak loads.
By the most stringent definition, a net zero building has a neutral carbon footprint, uses only renewable energy, and is net zero in terms of energy, water, and waste. Based upon this definition, it is hard to imagine a true net zero school that does not use some form of geothermal HVAC system.
How does geothermal work?
A geothermal system uses the ground or groundwater as a condenser or heat sink. There are two main types of geothermal systems: open and closed. In open systems, groundwater is pumped into a building, the HVAC system heats or cools it, and then it is pumped back into the ground. Closed systems are completely closed; that is, the same fluid pumps through the system over and over and never actually touches the ground.
Open systems suffer from mineral and scale buildup in piping and potential U.S. Environmental Protection Agency issues in pumping huge quantities of water from the ground. If the water is not pumped back to the same aquifer from which it is drawn, there may be contamination. For these and other reasons, most commercial systems use closed system designs.
The most common closed system uses vertical wells. A 6-in. diameter borehole is drilled 300 to 450 ft deep, a closed loop of pipe is inserted, and then the hole is filled with thermal grout.
These wells are usually spaced 20 ft from center and provide 1 to 3 tons of heat rejection per well. A typical building will require approximately one acre of wells for every 65,000 sq ft of building floor area. The wells are connected together in circuits consisting of five to seven wells.
The circuits are individually run back into the building where they are connected to large supply and return distribution headers. This arrangement serves several purposes:
- It keeps pipe sizes down
- It limits the total well field capacity lost in case of a piping failure
- It allows the circuit flows to be equally balanced.
For large fields with more than 70 wells, instead of running 20 or more pipes back into a building, a series of vaults are used to house the distribution headers. This actually reduces costs, because the vaults can be centrally located in each field. These vaults are generally accessed only for initial balancing but may be required in case of a pipe break.
Geothermal wells provide a heat sink that any water-cooled air conditioner or water source heat pump can use. Conventional chillers, air-cooled packaged variable air volume (VAV) units, water-to-water heat pumps, water-cooled computer room units, and water-source heat pumps can all use a geothermal condenser water loop.
In a large well field, the ground cannot transfer energy fast enough to return to its normal temperature and therefore acts as a thermal storage device. This means that in the summer when the building is rejecting heat, the well field warms up. In the winter when the building is absorbing heat, the well field cools down. In a perfectly balanced building, the well field ground temperature is a sine wave with the peak temperature at the end of summer and the lowest temperature at the end of winter.
Most buildings, however, are either cooling- or heating-dominated depending upon location, schedule, and use. If another source of heating or cooling the well field is not provided, the ground temperature will increase or decrease out of its normal range, reducing HVAC efficiency, until the ground temperature becomes too extreme for the equipment to function at all. This rise or fall in the well field temperature can occur very quickly (within a year) or gradually over years or even decades. In fact, one of the first large geothermal fields (1750 tons) was installed at Stockton College in 1993 and experienced a gradual well field temperature increase until 2005 when a cooling tower was added.
In order to meet the most stringent definition of a net zero building, the building must have net zero energy and water usage, and waste production.
Fossil fuels are nonrenewable resources that produce hazardous emissions. Electric resistance heating and heat pumps use only electricity (which can be produced from renewable resources such as photovoltaics and wind turbines) and produce no emissions. Furthermore, heat pumps are the only heating systems that have efficiencies of more than 100%; they can be as high as 300% to 500% efficient. Even biomass, cogeneration, and trigeneration plants produce emissions. Unless the facility uses a process that creates waste heat, heat pumps are the only high-efficiency source of heat that does not produce emissions.
On the cooling side, evaporative cooling offers extremely high efficiencies but uses significant quantities of water. Any system with direct air cooling takes a huge hit in efficiency due to the high air temperatures at the same time that the internal loads are peaking.
The use of a closed loop geothermal system offers significant advantages to both heating and cooling systems. A closed loop system acts as a long-term heat sink and thermal storage system without using any water after initial filling. Its natural temperature is perfect for maximizing refrigeration equipment efficiency for both heating and cooling.
Because the well field acts as thermal storage, it can be recharged during off-peak periods or when the charger is most efficient to operate. Recharging the well field refers to the use of an external heating or cooling source to gradually raise or lower the average temperature of the ground surrounding each well.
In a heating-dominated building, a small quantity of direct solar heating panels (with efficiencies of up to 90%) could be used to slowly heat the well field throughout the summer as well as in the winter.
Geothermal design strategies
The real challenge to geothermal systems is minimizing the cost of digging the initial wells. There are several ways to approach this, including minimizing building loads, minimizing start-up costs of the other pieces of the HVAC system, and applying existing technology in new ways.
With well fields costing $3,000 to $6,000/ton, the advantages of reducing the building loads are obvious. Since any water-cooled system can use a geothermal condenser water load, the well costs are almost completely added first costs of $7 to $15/sq ft. Some of the strategies used to reduce the building loads include using outside air energy recovery, daylighting, and building orientation. At Zankel Music Hall at Skidmore University, EwingCole used two outside air units that dropped the peak load by 25% and reduced the simple payback period from 35 to 16 years. Because geothermal fields are guaranteed for 50 years and utility costs will surely rise, this geothermal system will have a low overall lifecycle cost.
Another strategy to reduce well field size for cooling-dominated buildings includes the use of closed-circuit coolers or dry coolers to meet peak demand loads and reduce the well field to meet the average load. At Command Control/Communication Network Transport (C2/CNT) East Facility, Aberdeen Proving Ground, Md., a 500,000-sq-ft computer lab that EwingCole designed for the U.S. Dept. of Defense, the company provided six well fields totaling 750 tons, and two dry coolers totaling 750 tons, to meet a peak cooling load of 1250 tons with N+1 redundancy. This halved the number of required wells, resulting in an initial cost savings of nearly $3 million.
At C2/CNT, different HVAC technologies were combined to reduce the first costs while still providing the high efficiencies associated with traditional geothermal systems:
- Packaged water-cooled VAV units were provided to serve the interior zones.
- Ceiling-mounted horizontal heat pumps were provided for the exterior zones.
- Water-cooled computer room units were provided for IT rooms and closets.
- Water-cooled outside air units with energy recovery were used to provide fresh air.
The building shell is heated and cooled by the heat pumps located in the exterior zones. Reheat was provided using series fan powered VAV boxes. To prevent overcooling, the air conditioning units have supply air reset based upon return air temperature. By eliminating hot water heating and reheat, and reducing control points by utilizing packaged air conditioning units, the total mechanical first costs were further reduced to only slightly higher than a conventional chiller/boiler/air handling system. Payback occurred within 5 years.
Net zero DoDEA schools
Because geothermal systems have the lowest energy usage and lowest carbon footprint when combined with renewable electricity, every school that is attempting to attain net zero should employ some form of geothermal technology. If geothermal does not currently have an attractive lifecycle cost or if the first costs are too high, partial hybrid systems should be installed. If even hybrid systems are out of reach, the building and HVAC systems should be designed with an eye toward future conversion.
In order for true net zero DoDEA schools to be produced, integrated building design must evolve into a team approach among the users, planners, architects, and engineers, and everyone must come to the table with the twin goals of meeting net zero and maximizing the learning experience.
Net zero requires an investment in the future. It will not come cheaply, but the return will be high. True net zero requires a new way of thinking beyond buying green energy carbon credits, and playing the U.S. Green Building Council LEED game.
Net zero requires a real redefinition of priorities and gives new meaning to integrated building design. System selections can no longer be based entirely upon the lowest lifecycle or utility costs. A new system of rating a building’s performance must be based upon the lowest total energy and water use with the least use of nonrenewable energy. The Energy Star Target Finder program may be the place to start in order to evaluate actual performance versus current code minimums.
Jarema is principal and a mechanical engineer with EwingCole in Philadelphia. His work experience includes high-efficiency HVAC system design for museums, academic facilities, laboratories, and other projects for the government.