This 90-yr-old building will become a great representation of sustainability, innovation, and energy savings for governmental buildings as it cultivates several federal and national standards and initiatives. Those include the ARRA high-performance and green buildings mandates as spelled out in Executive Order 13514, and the General Services Administration requirements of 20% less energy usage than its baseline usage in 2003. However, GSA-Region 4 has more stringent energy goals. In order to meet ARRA and Region 4 requirements, the project team utilized some innovative solutions to recover energy and water and create a high-performance building.
The existing two 220-ton chillers, which are near the end of their useful life, use HCFC refrigerant and are linked with a primary-secondary pumping system. The project’s team decided to replace these chillers with new centrifugal chillers using HFC-134A refrigerant with maximum 2.72 lb refrigerant per ton to earn the additional LEED® refrigerant point. HFC has zero ozone-depletion potential (ODP) and low global warming potential (GWP).
The new centrifugal chillers are equipped with three-stage compressors and VSDs to achieve 0.59 kW/ton peak load efficiency and an NPLV of 0.351 kW/ton. The maximum kW/ton at 50% load is 0.310 kW/ton. The chilled-water loop is reconfigured to be a variable primary loop with two variable-speed pumps (330 gpm each). A bypass with flowmeter is provided to maintain minimum flow in the chillers as required by the manufacturer. The chilled water temperature difference across the chillers’ evaporators is designed for 16°F. Higher temperature differential provides less water flow and less pumping power, resulting in improved overall system energy efficiency.
The condenser water system consists of two existing induced draft, counterflow cooling towers with two-speed fans. Two condenser water pumps are provided (660 gpm each). The fans will be changed to VFD to make their operation more efficient. Also, the towers will be relocated due to existing problems with plume drift staining parts of the building.
The chilled water system is provided with waterside economizer in lieu of an airside economizer. The decision to use the first option was made early in the design phase, as the existing building does not have sufficient shafts to accommodate bigger ducts of outside air in addition to its historic status, which made it even more difficult for the team to place an outside air duct through an exterior wall. Additionally, the airside economizer would have required large relief air ducts and associated louvers and controls, creating unresolved complexity to the architectural planning, and it would have harmed the historic status of the building. Other potential IAQ complexities were also avoided by choosing an waterside economizer.
The waterside economizer cycle is achieved by means of a plate-and-frame heat exchanger and automatic valves for changeover when outside air conditions permit. The water from the cooling towers is not, and should not be, circulated through the chillers because of concerns about fouling the system. The plate-and-frame heat exchanger consists of multiple plates adjoined together with gaskets and bars and was selected based on low pressure drop, which was 11.5 ft of water. Typically 10 to 15 ft of water pressure drop is preferred.1
The chilled-water system is equipped with a side-stream 90-ton heat pump chiller, which will precool the chilled-water return to the new chillers. The heat rejected from the chiller is used for preheat in the summer and in mild weather conditions. The heat pump chiller setpoint is 95˚ when outside air temperature is 85˚ or above, and will be reset to 115˚ when outside air temperature is 55˚. The heat recovery chiller connects the chilled water return pipe to the evaporator side, and to the hot water return pipe on the condenser side. See Figure 1 for the chiller plant schematic. Life cycle cost analysis predicted the pay back to be six years for this chiller, which is considered acceptable considering the 20- to 25-yr life of the chiller.
The energy savings from using a heat recovery chiller is due to elimination or reduction of energy used by the cooling towers and associated pumps, as well as to the elimination or reduction of energy used by the boilers during low heating requirements. Additionally, the dedicated heat recovery chiller reduces emissions of CO2 and other toxic fumes and gases released from the gas-fired boilers. It should be noted that dedicated heat-recovery chillers are used for the sole purpose of utilizing wasted heat from the building and not for air conditioning and process needs.
OCCUPANCY SENSORS IN STAIRWAYS
Stairways don’t have to be lit 24/7. A study conducted by GSA Region 4 has found that stairways in a typical building are occupied only 2 hrs per 24 hrs. The potential of energy savings is tremendous.
On the Vance project, the project team decided to explore the use of occupancy sensors to control the stairway lighting in the four stairways. The expected energy savings are approximately 22,336 kWh/yr, which is $2,234 annually based on $0.10/kWh. The assumptions are based on 48 new fixtures (four stairwells with 12 landings per stairwell) at an average cost of $250 per fixture including the occupancy sensor, and saving 83% of the current stairwell annual energy usage. The expected pay back is around five years.
In unoccupied mode, the lights will dim down to 1 ft-candle lighting level at the landing in accordance with NFPA-101, however, the full occupied mode requires lighting level to be 10 ft-candles in compliance with the P-100, a GSA standard.
This strategy is under further study, and was not part of the energy model until the writing of this article. This strategy, if funded, will add up to the efficiency of the Vance project.
DOMESTIC SOLAR HOT WATER HEATING SYSTEM
According to the Executive Order 13514 as well as the Energy Independence and Security Act (EISA-2007), Section 523, the building shall meet at least 30% of the hot water needs through the installation of solar hot water system. The Vance project is designed to meet 50% of its domestic hot water needs using two solar thermal panels. This system is expected to last more than 15 yrs beyond its payback period.
Solar thermal collectors capture energy from the sun and transfer it to the working fluid (water) efficiently. The heated working fluid circulates through a heat exchanger and consequently heating the domestic water, which can be stored in a tank or used directly.
The solar heating system is integrated with gas-fired condensing hot water heater, which has up to 98% thermal efficiency with 5:1 turndown firing range. Figure 2 illustrates the integration of the solar system with the gas-fired heater.
HOT WATER HEATING SYSTEM
The hot water heating system consists of two gas-fired condensing boilers, with thermal efficiencies up to 99% and with less than 20 ppm NOx levels. The boilers’ heightened efficiencies are driven from the low return water temperatures and from the use of a single pass condensing heat exchanger, which is made up of vertical water surrounded tube nest of high-efficiency tubes. The tubes’ inner surface is finned and is constructed from aluminum alloy. This inner tube is the fireside, and it is fitted within an outer stainless steel tube. The internal fins increase the heat exchange surface as well as they create channels to increase turbulence and heat transfer.
The boilers are connected with a VFD pumping system and will provide a supply hot water temperature of 115˚ whenever the outside air temperature is 55˚ or less, and it will provide 140˚ hot water whenever the outside air temperature is 25˚ or less.
WATER SAVINGS AND CHEMICAL-FREE WATER TREATMENT
The project team decided to push the limits of water savings as well. The team decided to use a chemical-free cooling tower water treatment, an alternate technology that uses amplified magnetic water conditioning to reduce water, sewer, and energy costs and eliminate toxic chemical usage Additionally, it maintains scale-free water and stops corrosion, which results in extended equipment life and reduced maintenance costs. The best aspect of this technology is that it is all natural.
The technology changes the negative static charge on the water molecules to positive charge due to the current being generated by the moving water (Faraday’s law). Some of the water molecules will separate into Hydronium (H+) and Hydroxol (OH-) ions. The pipe is grounded, which makes its surface negatively charged attracting H+ ions, which prevents scale and reduces it in the existing system. The OH- ions are repelled by the negatively grounded pipe, so corrosion is prevented.
Based on calculations, the chemical-free water treatment for this project is designed to save 947,223 gal of makeup water as well as 852,615 gal of sewer bleed water annually. The return on investment is around 1.28 yrs.
In addition to the use of low-flow plumbing fixtures, the project is harvesting rainwater and collecting it in an underground cistern system. The recovered water will be used to flush toilets and urinals as well as for makeup water to the cooling towers. The rain water re-usage will help the building save about 393,000 gal/yr. A 43,200-gal storage tank is used to support this strategy. Water savings are expected to be 58.5% with respect to the baseline case (FY04-08 average).
OTHER KEY INNOVATIVE ENERGY-SAVING FEATURES
In addition to the strategies mentioned above, several innovative strategies have been adapted, which have a direct effect on the cooling and heating loads of the building.
The new roof will have insulation with R-value of 30. High performance glazing will replace existing glass at the back of the building on the first floor, with a U-value of 0.35.
The rest of the windows will be provided with low-E film on the interior of the glass, and daylight harvesting will be implemented in most spaces with exterior windows.
The project uses a single large DOAS ventilation unit for 72% of the outside air required, and two other smaller units to decentralize the outside air load from the space loads. The number of these units has been dictated by the architectural layout of the building. The larger unit utilizes a runaround heat recovery glycol loop, while the smaller two units utilize energy wheels to recover energy from the exhaust air. Additionally, demand controlled-ventilation is used in densely occupied spaces while occupancy sensors are used in less densely occupied spaces. The project team is awaiting funding appropriations to use submetering, which will confirm claimed operation efficiencies as well as provide feedback to the building manager to facilitate continuous improvements.
GSA had two goals for this project to achieve. One was to design a building that uses 20% less energy than its usage in 2003. This project team achieved 49% less energy than the 2003 consumption, almost achieving 2.5 times the goal and 31% less than ASHRAE-90.1-2007. Additionally, GSA Region 4 has a goal of achieving 44,000 Btu/sq/ft/yr. The energy modeling results for both baseline building and the design case buildings are shown in Table 1. With the building area of 180,000 sq ft, the energy use index of 37,231 Btu/sq ft/yr is achieved. It is also noted that this project achieves 58.5% water savings.
The innovative strategies presented in this article, combined with other sustainable architectural strategies and occupancy controlled lighting, should enable this project to attain LEED Gold certificate under the 2009 (V.3) version while exceeding regional and national energy requirements. ES