The new S. J. Quinney College of Law building on the University of Utah campus features a combination of innovative ideas to form one original, state-of-the-art engineered system. At the foundation of the mechanical system, and quite literally at the foundation of the building, is a 375,000-gallon thermal storage tank. Unlike most stratified thermal storage tanks, the Quinney Law storage tank is short, only 12 ft deep, and is designed around a very small temperature difference of only 4°F. The temperature difference is small because the tank is designed to be cooled by irrigation water.
In addition to a shallow irrigation-cooled thermal storage tank, the building features a chilled beam system that allowed the floor-to-floor height to decrease from standard VAV system requirements, reducing the building height to just under high-rise classification, saving the project around $500,000 in reduced envelope and mechanical costs.
Initially, the owner desired to use the building’s landscape irrigation as a small supplemental source of cooling. Due to a small landscaped area, and water conservation practices in the landscaping design, the landscape flow rate was extremely limited. On the other hand, the irrigation wells for the entire 1,534-acre campus were located in a neighboring parking lot, and the main campus irrigation supply line was adjacent to the building.
The main irrigation lines are active every summer night from 8:00 PM to 10:00 AM, and irrigation flows as much as 1,000 gpm of 54°F water during that time. Irrigation flow accounts for over 2,300 ton-hrs of cooling capacity on peak watering days, and coincidentally, the calculated peak cooling load day for the building is just under 2,300 ton-hrs (194 tons of instantaneous peak load). As long as we could accommodate relatively warm water and a limited temperature difference (4°F), the irrigation system for the campus was a perfect match for the new building.
One concern was what the effect on the landscaping would be if the irrigation water were warm. We found research showing the plants like warm water, but in the end, we decided it didn’t matter because the university uses spray irrigation through impact heads or gear-driven sprinklers. Spray means that the water is allowed to equilibrate with the outside air, naturally reaching the wet-bulb temperature before it lands on the vegetation. It’s just like a cooling tower.
The irrigation water couldn’t lose pressure because we were using it for cooling, so we pump it from the irrigation line through a heat exchanger and back into the irrigation line as a side-stream. The law building does not consume any of the irrigation water, and there are no significant pressure losses in the main.
Using irrigation for cooling meant that we needed a way to store the cool temperature from the irrigation system for use during the day. The building’s site is sloped in such a way that the first floor exits at grade on the west side of the building, while the second floor exits at grade on the east side of the building. This architecture provided the perfect opportunity to tuck a tank on the east end of the first floor, directly below the second floor, and incorporated into the structure of the building.
A thermal storage tank on one typical level of a building means that 12 ft is the maximum depth we could obtain. It also means that if the tank needs to be any significant size, structure will have to pass through the tank to hold up the building above. Both of these conditions held true, and presented their own unique set of challenges.
The idea of a stratified thermal storage tank is that cooler water is introduced at the bottom of the tank at a low velocity, and because of density differences between cold and warm water, the water does not mix. However, even if the convective mixing is controlled perfectly, conduction still occurs from the warm layer above to the cold layer below, creating a temperature gradient between the two layers. The gradient that forms between the two layers is referred to as the thermocline. In ideal circumstances, the thermocline is about 3 ft deep.
The thermocline in a thermal storage tank is useless water to the system. It is not cool enough to use for cooling, but it is not the return temperature either, meaning it hasn’t been used up. Having 3 ft of unusable thermocline in a 12-ft deep tank means that 25% of the tank is unusable.
With concrete columns and structural walls running vertically through the tank, the decision was made to split the tank into four separate compartments. That way, a 3-ft thermocline only is taking up 1/16 of the total usable volume. But, how do we split a shallow tank and still maintain an unmixed condition with stratification and a thermocline?
Computational Fluid Dynamics
CFD was used to verify the design. A variety of options were simulated, and ultimately, the best solution was to use perforated horizontal baffles to prevent unequal, or sudden, high vertical velocities in the tank, and evenly distributed arrangement of pipe inlets and outlets. This showed the least amount of mixing with the least cost. The design for each compartment includes eight cold water inlets directed downward, 6 in above the bottom of the tank, flush with a stainless steel grating with 15% open area. The top of the tank is similar, with eight return water inlets directed upward, 6 in below the water surface, also mounted flush with a perforated grating with 15% open area.
As a final test to the design, the results at the pipe outlets, in the CFD model, were used as the inputs at the pipe inlets for a new CFD model. The outputs from the second CFD model were inputs for a third, and the outputs from the third were inputs for a fourth. In that way, all four compartments were modeled to identify any performance issues as the thermocline progressed through the tank. Figure 2 shows the outlet temperatures for each of the four compartments. The sudden change in temperature is where the thermocline is passing through the outlet. A consistent slope indicates a uniform thickness of the thermocline.
Pipe Flow Analysis
For water to flow from one compartment of an atmospheric tank to the next, there has to exist a height difference between the levels of the two tanks. In this case, the tank compartments are connected by a series of pipes. So, the pressure drop due to velocity flow in the pipes has to be balanced with the desired height difference in the compartments.
Since we are dealing with a shallow tank, the height difference between compartments has to be small. The pipe sizing and layout were sized in order to require less than 4 in of WC pressure drop between each compartment at 1,000 gpm flow.
The pipes are laid out in a self-balancing H configuration. The configuration was modeled in AFT Fathom pipe flow analysis software, and elbows, tees, and pipe sizing were adjusted to meet the 4 in WC pressure drop requirement. Large-radius, small angle elbows, oversized pipe, and smooth polypropylene were used to meet the goal.
Whenever the irrigation pumps are running, the control system tests the temperature of the irrigation water. If it is cool enough to charge the tank, and the tank needs charging, pumps change out the water in the tank with cool water.
The charge-level of the tank is determined by 40 temperature sensors placed in the four tank compartments. Because each compartment has a different area, the sensors are spaced differently from compartment to compartment, so that each temperature sensor represents an equal volume of water. If a sensor is below the charge temperature, then its volume of water is considered charged. However, the total amount of the tank charged is based on how many consecutive sensors show a charged state moving from the cold water inlet to the warm water outlet.
As needed, the building pumps move water from the tank through the chilled beams and back into the top of the last compartment. Since the tank is an open system, at the bottom of the building, a pressure sustaining valve was installed to prevent the building from draining back down into the tank and overflowing the tank when the pump is off.
A chilled beam system seemed to be a perfect pairing for the warmer than normal supply water temperatures. The beams were designed to maintain space setpoint with 56°F cooling supply water as long as the humidity outside stays in check. In Salt Lake City’s relatively arid climate, high humidity is only a concern a few days in the year. On those days, dehumidification is provided at the air handler with chilled water from the campus central utility plant.
The first thing that air sees as it comes into the building is a run-around heat recovery coil, that recovers heat (and cool) from the exhaust air stream. The air then passes through filters, a pre-heat coil, fans, a cooling coil, and finally a separate dehumidification coil.
The dehumidification coil not only provides humidity control during the muggy summer days, it also provides a way to pre-heat cold outside air while simultaneously charging the tank in the winter. During shoulder seasons, irrigation may be off, but during cool nights, the tank can recharge by running the air handler and cooling down the thermal storage tank using outside air.
If the tank is going to run out of cooled water before irrigation turns on, and it is warm outside, the system turns to campus chilled water as a backup source of cooling. There will be times when irrigation requirements do not match up to the building load requirements and chilled water will be needed.
The building features operable windows in many of the faculty offices. The controls system shuts the control valves in any offices with windows open, preventing the unnecessary use of energy to condition the outside environment. Occupancy sensors throughout the building control the two-position VAVs, giving demand controlled ventilation everywhere.
Energy Model and Life-Cycle Cost Analysis
An energy model was created for the entire project to evaluate the potential cost savings of multiple proposed energy efficiency measures (EEMs). The project was modeled using Trane Trace, and followed ASHRAE 90.1-2007 Appendix G guidelines
The thermal storage tank is, by nature, a mostly maintenance free system with few moving parts. The savings in operations and maintenance costs compared to a typical water cooled chiller with cooling tower are considerable. Additionally, the project did not need to locate a cooling tower onsite, potentially obstructing views or creating a noise nuisance.
Results of the analysis predict that the thermal storage tank will save approximately $17,000 per year in energy costs.
Phil Jankovich, Ph.D., P.E., is a project engineer at Colvin Engineering Associates. With degrees and experience in Chemistry, Chemical Engineering, and Mechanical Engineering, Phil’s diversity of knowledge allows him to consult in many areas. Phil’s dedication to energy efficiency and conservation is evident in all he does. He has commuted by bicycle and public transportation for the last 15 years.
Stephen Connor, P.E., is the president of Colvin Engineering Associates. Mr. Connor has been a local leader in sustainable design. He was the first engineer in Utah to be LEED accredited and has worked on over 40 LEED-registered or certified projects. He has served the local chapter of the U.S. Green Building Council, is a past-president of the local ASHRAE Chapter, and has served on the Salt Lake City Mayor’s sustainability advisory committee.
Matt Wilson is an ASHRAE Building Energy Assessment Professional, and LEED AP with nine years of whole building energy simulation, and mechanical design experience, at Colvin Engineering. He has extensive practice performing and reviewing energy models for LEED, Energy Star, EPAct, DFCM HPBS, life-cycle cost analysis, and various utility incentives, as well as extensive experience consulting projects on energy code compliance issues. He is skilled with a wide-range of building analysis and exceptional analysis tools, including computational fluid dynamics (CFD).