FIGURE 1. Sump water schematic. Sump pump operation is intermittent, based on ground water level, but the surge tank allows the transfer pump to run continuously. The heat exchanger allows chemically treated building circulating water to be segregated from the sump water.
When George Washington Carver School was built in 1935, boiler room excavation inadvertently intercepted an underground spring. Ever since, sump pumps ran continuously to remove approximately 150 gpm of ground water from the school’s basement. This is the story of using imagination, a central reversing chiller, and smart pumping to forge a sustainable solution that would make the school’s namesake proud. Adding cooling while reducing overall energy costs significantly made school officials pretty happy, too.
When George Washington Carver Elementary School in Indianapolis (#87) was built in 1935, an underground spring was inadvertently intercepted by the boiler room excavation. Ever since, sump pumps have run continuously to remove approximately 150 gpm of ground water from the school’s basement. For 70 years, the ground water was seen as a significant liability, since several power outages had disabled the sump pumps and flooded the boiler room.
In 2006, Indianapolis Public Schools (IPS) chose school #87 to be a year-round, inner city magnet school, requiring the addition of air conditioning. IPS sought out a consultant who had a vision to use the ground water for the building’s heating and cooling system. The design now provides cooling at half the cost of conventional equipment, and heating for about one quarter the cost of the old system. The ground water is now a significant asset. The creative legacy of George Washington Carver continues at his namesake school.
The Design
“Since new developments are the products of a creative mind, we must therefore stimulate and encourage that type of mind in every way possible.”- George Washington Carver
Geothermal heating and cooling will almost always be more efficient to operate than conventional systems. The first-cost premium is usually the cost of drilling the wells (bore holes), which may be as much as $3,000 per installed ton of cooling. In urban settings like this one, there is seldom enough acreage for a well field (bore field). For example, at nominal well spacing of 20 ft, a football field would be big enough for about 350 tons of cooling, given reasonable subsurface conditions. (The subsurface can be highly variable, and appropriate design discretion must always be exercised.) At IPS #87, the geothermal water was already there, waiting for a “Carver-esque” engineering solution.
Figure 1 shows the arrangement of the sump pumps, storage tank, transfer pump, and heat exchanger. All components are backed up with 100% redundant spares. The ground water is pumped into a 10,000-galstainless steel storage tank that overflows to the building sewer, just as it has for 70 years.
Transfer pumps route the ground water through a set of 100% redundant heat exchangers that allow building circulating water to be segregated from the ground water. The tank is designed so that the transfer pumps see the most efficient water temperature in both the heating and cooling modes. And the tank is designed with both inlet and outlet diffusion headers that allow the tank to stratify with the cooler water at the bottom. The sump pumps are on an emergency generator, since power failure would flood the boiler room in about a half hour. The sump has multiple high level alarms.
Rather than multiple distributed compressorized units throughout the building (conventional geothermal heat pumps), #87 has a single unit located in a central mechanical room. Two concerns dictated this approach vs. water-to-air heat pumps:
Heat pumps cannot handle cold winter air or humid summer air effectively, and because of that, most heat pump buildings are equipped with decoupled makeup air systems, a big first-cost penalty. The makeup air is frequently a high-hp fan, an operating cost penalty.
Because they cannot handle outside air, heat pumps cannot run economizer cooling, which is a significant operating cost penalty in a school.
FIGURE 2. Building water schematics. The building EMS will decide automatically what mode is most efficient, based on room temperature, outside temperature, outside dewpoint, and tank temperature.
The GEO-H/C is connected to a “modern” two-pipe building system, defined as a two-pipe that will change over quickly and automatically, and as often as weather dictates. The airside equipment is standard air handlers, unit ventilators, and fancoils.
Figure 2 shows the three water flow arrangements for the building system. These schemes have two-position, three-way valves to reverse the water flow from heating to cooling, and two-way, two-position valves that open for waterside economizer (sensible cooling) operation. Since the ground water temperature is a relatively constant 55° to 58°, there are significant times of the year which would be suitable for sensible cooling, if the outside air dewpoint is low enough to avoid elevated space relative humidity. The sensible cooling mode is also an energy-free way of cooling the building loop when the two-pipe system changes over from heating to cooling.
The two-pipe classroom units have face and bypass dampers in lieu of control valves, and other terminal equipment utilizes single zone VAV, fan speed modulation, and outside air management techniques to improve both energy efficiency and summer humidity control.
The GEO-H/C operation is a critical aspect of the design. In cooling mode, things are relatively simple. The chilled water setpoint is fixed and compressors stage off and on as required to meet the load. The transfer pumps run at a slow speed to ensure that the GEO-H/C condenser cooling water isn’t too cool.
In heating mode, things are significantly more complicated. The building supply water temperature is on a reset schedule vs. outside air temperature (OAT), such that at 60° OAT the hot water supply (HWS) temperature is 90°. HWS is reset up to 130° HWS at 10° OAT or colder. The initial GEO-H/C heating setpoint (90° HWS at 60° OAT) would be with:
- The building pumps connected to the condenser side of the GEO-H/C, running at a constant speed
- Normal evaporator setpoint, 44°
- Varying the speed of the tank transfer pumps to create enough load to raise the condenser temperature to the desired level
Although not shown on the diagrams, IPS asked that emergency heating boilers be provided in the design, since a GEO-H/C outage could leave the building without heat. During the winter of 2006-07, this proved to be a wise decision, since the boilers did run on several occasions until the GEO-H/C control sequences were refined. It is hoped (expected) that future winters will not require boiler operation. The boilers are piped so that they can supplement the GEO-H/C output or operate independently.
TABLE 1. Twelve-month total operating costs for #87. The “before” numbers are based on a nine-month school calendar without cooling. The “after” numbers reflect a year-round school calendar with all spaces air conditioned.
Energy Efficiency, Environmental, and IAQ Impacts
“When you do the common things in life in an uncommon way, you will command the attention of the world.”- George Washington Carver
Heating impact.When it was built, #87 was heated by coal fired, low-pressure steam boilers serving classroom unit ventilators and fancoils. At some time in the subsequent years, the boilers were replaced with 80% efficient gas-fired units. Another upgrade converted classroom terminal heating equipment to 180° hot water, and the boiler room was retrofitted with a steam-to-hot-water heat exchanger. Although no definitive measurements were made, it is estimated that the annual fuel utilization efficiency (AFUE) of the old system was in the 50% range, or a COP of 0.5 (gas). The new system operates in heating at an average COP of 4.2 (electric).
When corrected for the price of the two energy sources, the new system operates in heating for one quarter the annual cost of the old system. The back-up boilers are high efficiency condensing units, the IPS standard, and the entire building heating loop is designed for low temperature heat, 130° max temperature. The heating water temperatures are lower than what had been used here, and lower than the “industry average” of 180°, resulting in significantly lower parasitic losses from piping, etc.
Cooling impact. Prior to this retrofit, the only parts of #87 that were cooled were the principal’s office, the cafeteria, and the parents’ room. IPS policy calls for adding cooling to all buildings as renovation budgets allow. The IPS standard for elementary buildings is the air cooled chiller with ARI rating of 1.25 kW/ton. The new system utilizing the GEO-H/C operates at a worst-case efficiency of 0.65 kW/ton and an average of 0.55 kW/ton. The GEO-H/C system requires an additional pump that a conventional primary/secondary system would not have, increasing the GEO-H/C kW/ton by 0.07.
Operating cost impact (Table 1). This project reduced gas consumption from 25,370 therms ($28,501) to 796 therms ($1,098), a 96.7% savings. Electric consumption was increased from $22,770 to $41,574, resulting in total utility cost of $51,271 before the renovation (no A/C, nine-month calendar) to $42,672 after the renovation with the entire building cooled (12-month calendar). This is a 17% reduction. The dollars have been normalized for the cost of energy from 2005-06 school year to 2006-07.
Environmental impact (Table 2). The attached table shows the combined air quality environmental impact of the project through 12 months. All numbers are pounds of pollutants, and they show a net increase of 29%, again while adding cooling and converting to year-round school. These numbers are calculated as “source impact.” Had #87 been cooled and heated conventionally, this building would have released more than 5 billion Btuh into the atmosphere.
TABLE 2. Total pounds per year of pollutants released as a result of utility use at #87. The electric column represents source generation pollutatns, as opposed to point of use. They are higher than might otherwise be anticipated because of the 35% average efficiency of power plants. (Reference: Electric Pollution Calculator, www.infinitepower.org).
IAQ impact. When #87 was converted from steam to hot water and the classroom units were changed (mid-1980s), the recommended fresh air quantity was 5 cfm/person. This renovation increased fresh air to 15 cfm/person to meet current the ASHRAE-62 recommendation and local code. The renovation revealed, however, that independent of the original O/A balance number, the students were probably getting reduced fresh air because most of the louver screens were blocked with leaves, grass clippings, candy wrappers, etc. This project implemented several measures to ensure that fresh air to the spaces will not be compromised in future years:
- The CO2 level in each classroom is monitored, and if elevated levels are detected, a maintenance order is issued
- Access panels were added to the outside air ducts to allow easy cleaning of the louver screens
- Summertime humidity levels are tracked, and no space exceeds 60% rh during occupied or unoccupied hours
- Space temperatures are monitored, and each teacher has limited control (plus/minus three degrees from a nominal 72° setpoint) over his space
- The new units are equipped with extended surface MERV-8 filters
The GEO-H/C is on the left, the building pumps (blue piping) and the HX pumps (green piping) are on the right.
Conclusion
“There is no short cut to achievement. Life requires thorough preparation.”- George Washington Carver
The design objective of any school air conditioning retrofit should be that the total utility cost with cooling does not exceed the pre-renovation operating cost as a heating-only building. Measured in these terms, and probably any other terms, the upgrade at George Washington Carver School was an achievement. Dr. Carver would approve. ES
Sidebar 1: Internal vs. External Reversing
The GEO-H/C is a non-reversing refrigeration machine as the system reverses externally. The GEO-H/C evaporator will always be the evaporator and the condenser will always be the condenser. Both will always be counter flow heat exchangers, significantly more efficient than parallel flow heat exchangers.A water-to-water heat pump typically reverses internally. The manufacturer has to decide which mode (heating or cooling) to optimize. When a heat exchanger can be either a condenser or an evaporator depending on the mode of operation (heating or cooling), there will be counter flow between refrigerant and water in one operational mode (good heat transfer) and parallel flow in the other (poor heat transfer). Parallel flow can increase the heat exchanger’s approach by as much as 10°F, equal to a 30% impact on EER. The refrigerant side pressure drop of a four-way valve adds a minor inefficiency that does not exist with externally reversing equipment.
Although not a part of the #87 design, the GEO-H/C can provide both chilled and hot water concurrently on four-pipe systems, which cannot be done with most water-to-water heat pumps.
Sidebar 2: George Washington Carver: An American Renaissance Man
George Washington Carver was born a slave, and later, as a free man, earned his bachelor of science degree from Iowa State University in 1894.In 1896, he joined the staff of the Tuskegee Institute as director of the department of agricultural research, retaining that post the rest of his life. His work won him international repute. Carver’s efforts to improve the economy of the South (he dedicated himself especially to bettering the position of African Americans) included the teaching of soil improvement and of diversification of crops.
He discovered hundreds of uses for the peanut, the sweet potato, and the soybean, and thus stimulated the culture of these crops. He devised many products from cotton waste and extracted blue, purple, and red pigments from local
clay.
From 1935, he was a collaborator of the Bureau of Plant Industry. Upon his death in 1943, Carver contributed his life savings to a foundation for research at Tuskegee. In 1953, his birthplace in Diamond, MO, was made a national monument.
