EARTH TO SYSTEMS - GEOTHERMAL SYSTEMS

Energy

Written by Anthony Martin, P.E., LEED AP

Asking whether the US Green Building Council’s LEED Rating System views energy as a high priority is like asking a 3-yr old whether or not they want chocolate cake for dessert, you already know the answer.  Or put another way, Energy within LEED is like the peanut butter in a peanut butter sandwich, it just wouldn’t be the same without it nor would it even be considered a peanut butter sandwich. These are just a few analogies that can be made when LEED and Energy and leaves us asking a few more questions: Why such an importance?  What do we do about reducing our needs?  How do we prove that we are meeting our energy goals?

IMPORTANCE OF ENERGY IN LEED

If you have spent any time reviewing the new Version 3 LEED Rating system, you have undoubtedly seen the new distribution of points towards certification.  Recently the USGBC hosted a series of webinars to introduce these changes and point out why the changes came about.  One of the main driving factors was to better weigh the points involved with the rating system to better influence the variables that affect the climate change within our environment.  We can all agree that creating, distributing and using energy greatly affects the carbon foot print of the building.  Therefore, when speaking of Energy usage, any step to reduce our need on Energy has basically a one for one connection with reducing the climate change variables associated with our new building construction.

But that’s the green building industry’s take on energy usage, what about the end users point of view, the building Owner.  Buildings are built for one driving purpose; they fill a need that in turn fills a pocket book.  As energy rates continue to rise, the need for more efficient buildings are on the rise as well.  It boils down to a very elementary equation; money coming in must be higher than money going out.  It’s easy to see that reducing energy costs has a direct effect on the profitability of the building owner.

Reducing the Energy Burden through Geothermal Heat Pumps
But that is merely making the point as to why Energy is the peanut butter of the LEED sandwich, how do we reduce our needs on energy usage?  Energy modelers and the MEP design community are quick to point out that energy begins and ends with the HVAC and lighting systems that we design into the buildings.  In fact studies have shown that approximately 40% of the building’s energy needs are from the requirements to heat and cool the building.  Therefore, if we can improve our HVAC equipment efficiency and building systems, then we can reduce our energy costs.  So what HVAC system is right for me?

Recently, the North Texas Chapter of the USGBC hosted a presentation on geothermal heat pump systems and their role with achieving LEED Energy and Atmosphere Credit 1 – Optimize Energy Performance and is just one of the many options that the MEP designer can choose to utilize to improve the efficiencies of the HVAC system.  These systems are not as complicated as one may think and operate much like the one found on a typical residence.

Let’s analyze the typical residential HVAC system as an example.  As air moves across the cooling coil located in the home, heat is pulled from the building air stream.  This heat is then transferred outside the building using a refrigerant where it can be relieved to a heat sink, the atmosphere.  Those familiar with the Texas climate know that two variables typically occur.  First, throughout any particular day, the outside air temperature can rise and fall about 20 degrees.  Secondly, we know that we need more air conditioning as the outside air temperature rises.   Our grade school lesson tells us that heat transfers from the hot body, the refrigerant, to the cold body, the atmosphere.  Additionally, the greater the temperature difference between these two bodies, the more efficient heat can be transferred.  Therefore, a rise in outside air temperature results in a rise of the “cold” body and causes a decrease in the efficiency to move heat.   

In short, the typical geothermal heat pumps (or better classified geo-exchange heat pumps) utilize this same philosophy to heat and cool the building.  The difference is that a constant temperature “cold’ body is now utilized to stabilize the ability to transfer heat and increase the efficiency of the HVAC system.  The “cold” body can take on many shapes and forms and is what classifies the type of geo-exchange system being used and the efficiency of the HVAC system.  
There are typically four types of Geo-Exchange systems utilized for commercial buildings: the open water loop, closed water loop (vertical and horizontal) and the pond/lake water loop.  Each system has its pros and cons and should be examined for both upfront costs and life-cycle energy utilization.  

OPEN LOOP GEO-EXCHANGE SYSTEMS

The open loop system is one that utilizes water derived from aquifers below the ground through a series of wells.  The water temperature of these aquifers is typically stable in the range of 68-78 degrees Fahrenheit depending on where the project is located.  Water is pumped from the ground into a heat exchanger where the heat pulled from the building side is transferred to the well water and re-introduced into the Earth.  The benefit to this type of system is that the heat sink is maintained at a constant temperature.  

Therefore, if a building is constantly trying to reject heat, something typical to the southern climate, a constant, cold heat sink will improve the equipment efficiency.  There are risks in implementing one of these systems that must be addressed.  The two most notable issues are the abundance of fresh water within the aquifer and the quality of water that can be extracted.  If water cannot be easily extracted, then additional wells may need to be addressed and additional pump energy will be used.  Additionally, bad water will tend to scale up the HVAC equipment, thus reducing the ability to transfer heat efficiently.

CLOSED LOOP GEO-EXCHANGE SYSTEMS

The closed loop system (vertical) is probably the most common geo-exchange system designed and utilized in a HVAC system.  Rather than pulling water up from the Earth, water is pumped down into the ground where it can transfer the heat energy to the surrounding soil.  Holes are drilled into the ground typically ranging from 150 to 300 feet and piping loop is installed.  The loops are then gathered and pumped back into the building where the heat load from the building can be pulled out of the spaces.  These systems are typically desired because they utilize a small footprint to be installed and can be hidden below potential other building elements such as parking lots, landscaping or adjacent other green spaces.  Additionally, the water quality issue is now avoided in the HVAC system because the water is not open to potential contaminants.  However, these systems can be relatively expensive to install ranging between $1,000 and $1,300 per ton.  Additionally the surrounding soil conditions need to be examined for the ability to move conduct heat from the pipe to the surrounding grounds.

The closed loop system (horizontal) is very similar in nature; however, instead of drilling holes into the ground, piping is laid at a relatively shallow depth of 5 to 10 feet.  The reduction in depth reduces the installation costs to roughly $600-$800 per ton; however, the same rough amount of piping still has to be used.  Because of this these systems are typically not installed unless a vast amount of space (approximately 2500 square feet per ton) can be allocated to the HVAC underground piping system.  Again, the conductivity of the surrounding soils must be examined to ensure that the heat can be transferred away from the site and avoid “well-field burnout.”

POND/LAKE CLOSED LOOP GEO-EXCHANGE SYSTEMS

The pond/lake closed loop system is a variation to the horizontal, below grade system noted above.  Rather than utilizing the earth as the heat sink, these designs introduce large volumes of water to move heat from inside to outside the building.  The water is piped out to a series of piping bundles or plate-frame heat exchanger where the heat can be transferred to the surrounding pond/lake.  Consideration must be given to the size of the water being “pumped” with heat as well as the amount of water movement naturally or mechanically available to ensure that algae buildup is minimized.  Think of a hot tub where heat is continually pumped into it, over time the hot tub would be unbearable.  Consider this with a large Olympic sized swimming pool and the same amount of heat being introduced, it would take a much larger time period to heat that volume of water.  These systems are typically the most cost effective solutions but they require a very important site amenity, a large volume of water.

ENERGY EFFICIENCY

PROOF FOR OUR ENERGY EFFICIENT DESIGNS
So if we agree that energy reduction is important and know that a geo-exchange heat pump system is a means and method towards improving energy efficiency, how do we prove that we are actually doing what we said we were doing by implementing such a system?  The USGBC LEED rating system looks at this very question through Energy and Atmosphere Credit 1 – Optimize Energy Performance.  Those familiar with this particular credit know that there are currently (4) options available to show compliance; however, when such a high efficient system such as geo-exchange heat pumps is designed, there should only be one option that should ever be considered – Whole Building Energy Modeling.

ENERGY MODELING DYNAMICS

Whole Building Energy Modeling is a prediction tool for estimating how much energy a particular building will use.  Each of the systems outlined above has its own challenges in modeling the systems that need to be addressed whether a geo-exchange system is utilized or not.  Therefore, addressing some of these key elements is required for any good energy model.

First and foremost the energy modeler must have a complete understanding of the Standard utilized for this particular credit, ASHRAE 90.1-2004 (or 2007 depending upon what Version of LEED being used).  Table G3.1.1B and corresponding sections of the Standard note that type of system to be modeled for the base building system.  These range from packaged, air-cooled, DX units up to chilled water systems with variable air volume air handling units.  

Those familiar with geothermal systems know that these systems are more efficient than any of the systems listed in this table.  In fact the whole system energy usage of the systems found in the baseline system range from 1.2 kW/Ton (chilled water) to 1.4 kW/Ton on the packaged DX equipment; whereas, a typical open loop system will  see whole systems efficiencies of 0.9 kW/Ton and closed loop systems of 0.88 kW/Ton.   Therefore, knowing the Standard and the type of equipment that will be compared against the designed system is vital for starting any energy model.

Secondly, scheduling and managing loads in the building is pivotal in accurately modeling the loads of any particular HVAC system especially geothermal systems.  Most MEP designers design buildings based on the worst case scenario, the building fully loaded at 3-4pm in the middle of the summer.  Energy modeling requires more refinement.  Again energy modeling is a tool to predict energy usage and therefore schedules must be implemented to predict how the building is going to be used.  

Additionally, designing for the worst case scenario of each space (sum of the peaks) is also going to drastically alter the size of the well field and the performance of the energy modeling.  Therefore, scheduling and “right sizing” of the HVAC equipment in the energy model must be followed.

Finally, the most important aspect of any HVAC system and particularly geothermal systems is the part loading performance curves of the equipment.  Those familiar with building operation or even weather patterns know that loads within a building change drastically over time.  Equipment manufacturers provide how the equipment will hold up and improve efficiency during these part load conditions.  Therefore, importance must be given into ensuring that a part loading curve consistent with the equipment designed.

For instance, ASHRAE 90.1-2004 requires that a 5-ton water source heat pump operating at 86 degrees Fahrenheit be rated at a EER rating (energy performance rating) of 12.0.  One manufacturer’s scheduled performance rating for that sized unit is 15.0 EER, an improvement of 25% over ASHRAE 90.1-2004.  However, the part loading performance at 70% total load of that same unit is 17.6 EER resulting in a 47% improvement over ASHRAE 90.1-2004.  Therefore, by ignoring the part loading performance curves, a 22% improvement over the baseline would never be seen by the energy model.  

ART AND SCIENCE

Energy reduction design and modeling is an art as much as it is a science, but the importance of energy reduction cannot be overstated.  Not only is it required to show a high level of LEED certification but it is important to hand of a building to an owner that can be run as cost efficient as possible.  Whether you design a geothermal system or one of the hundreds of other HVAC systems available, careful consideration of both the design and energy modeling is required.  But at the end of the day, we can make a difference in reducing the climate change through striving for energy reduction.