Abstract
Seemingly small efficiency improvements to traditionally designed chiller plants multiply to create impressive savings in district cooling plants. The size and scope of these larger projects make the benefits more obvious and very valuable. For example, the United Arab Emirates is fully embracing district cooling for accommodating its tremendous growth, rather than using individual cooling systems in each building. It has been estimated that 75% of the energy used in Dubai, U.A.E. is for cooling. By using district cooling, Dubai planners expect to reduce the amount of electrical energy used by 40%. More than 30% of the Dubai market is selecting pre-engineered, packaged chilled water systems, many of which employ series chillers, variable-primary distribution design, and other concepts discussed in this article.
INTRODUCTION
District cooling was inspired by district heating plants, which became popular in the 19th century for providing steam to a number of buildings and often to entire portions of cities. Recent district cooling projects exceed 100,000 tons, typically in urban areas with dense, simultaneous development. One project of this size can, all by itself, magnify an incremental efficiency improvement into nearly 10 MW and offset $10 million or more in power generation capacity.
These ideas can naturally be used on smaller projects, where the benefits are less immediately obvious. To illustrate the energy and cost savings potential, this entry includes an analysis from a 10,500-ton convention center. The key design elements used on these projects violate traditional rules of thumb and allow the system designer to focus on the energy consumed by the entire chilled water system.
MOVING AWAY FROM CHILLER-ENERGY-ONLY ANALYSES
In the last 40 years, water-cooled chiller efficiency expressed in coefficient of performance (COP) has improved from 4.0 to over 7.0. Cooling tower and pump energy has remained largely flat over the same period. The result is that towers and pumps account for a much higher percentage of system energy use.
Because of these dramatic improvements, it is tempting to go after chiller savings in new plants and chiller replacements. After all, the chiller usually has the biggest motor in a facility. Many chiller plant designs continue to use flow rates and temperatures selected to maximize chiller efficiency and ignore the system effects of those decisions. Chilled water system designs frequently default to using the flow rates and temperatures used for the rating tests developed by the Air Conditioning and Refrigeration Institute (ARI) standards 550/590 for vapor compression chillers[1] and ARI 560 for absorption chillers.
While these benchmarks provide requirements for testing and rating chillers under multiple rating conditions, they are not intended to prescribe the proper or optimal flow rates or temperature differentials for any particular system. As component efficiency and customer requirements change, these standard rating conditions are seldom the optimal conditions for a real system. There is great latitude in selecting flow rates, temperatures, and temperature differences.
Today, an equal price centrifugal chiller can be selected for less condenser flow, with no loss in efficiency. At the same time, chilled water temperatures are dropping. Pump savings usually exceed chiller efficiency losses. Larger chilled water distribution systems will realize higher savings. Analyze the entire system to define the right design conditions.
MARKET TRENDING TOWARD HIGHER LIFT
Chiller efficiency is dependent on several variables— capacity (tons) and lift (chiller internal differential temperature) are two of them. Lift is the difference between the refrigerant pressures in the evaporator and condenser, and it can be approximated using the difference between the water temperatures leaving the evaporator and condenser.
Designers are converging on higher-lift systems for a variety of reasons; e.g., more extreme climates, system optimization, replacement considerations, thermal storage, and heat recovery. Higher lift creates solutions for engineering problems and leverages chiller capabilities.
Chiller plant design for U.S. conditions usually calls for a maximum tower-leaving temperature of 85°F. Conventional designs used 44°F/54°F evaporators and 85°F/95°F condensers—easy on the chiller (and the system designer). But higher ambient wet bulb conditions are common in the Middle East and China. Many parts of China call for 89.6°F design condenser/tower water, and parts of the Middle East design for 94°F. As the cooling markets grow in areas with extreme weather, technology providers must find better ways to deliver cooling at the higher lift conditions.
A reduced flow rate in the condenser saves tower energy and condenser pump energy. In the replacement and renovation market, the resulting increase in delta-T (tower range) delivers the same capacity with a smaller cooling tower, or more capacity with the existing cooling tower.
Chilled water temperatures below 40°F allow system designers to implement a larger chilled water delta-T. The result is less costly chilled water distribution and more effective cooling and dehumidification. Besides reducing water flow rates, colder chilled water gives higher delta-Ts at the chiller and better utilization of the chillers.
Thermal storage is also becoming more prevalent. For stratified chilled water storage tanks, 39°F water is the magic number, because it corresponds to the maximum density of water and keeps the charged section below the thermocline.[2]
Higher condensing temperatures provide more useful heat to recover from the condenser water. Some commercial building codes require condenser heat recovery for applications with simultaneous heating and cooling.
A proper chiller selection can deliver these conditions with little or no extra cost. It is possible to increase lift while increasing overall chiller plant efficiency by using a series-counter flow chiller arrangement.
SERIES CHILLERS
In their heyday of the early 1960s, series chillers were widely used in government buildings in Washington, D.C. Series arrangements were necessary for perimeter induction cooling systems, which supply cold primary air to the space and require colder water from the chiller. As previously discussed, chillers in the 1960s had a COP of about 4.0, with high-flow (velocity) smooth-bore tubes, low tube counts, and one-pass evaporators to reduce pressure drop. The most efficient centrifugal chillers today have a COP of more than 7.0—more than 75% higher than chillers used in these early series chiller plants.
Designers virtually stopped using series arrangements in the 1970s because variable air volume (VAV) systems made the colder chilled water used for induction systems unnecessary. Given the chiller’s relatively low efficiency by today’s standards, it made sense to raise temperatures and save chiller energy. Variable air volume systems were widely adopted because they saved energy and adapted to unknown cooling loads. Variable air volume systems are still the most popular choice for comfort cooling applications.
Induction systems are virtually nonexistent in 2006, but the dramatically improved centrifugal chiller efficiencies are driving resurgence in series chiller arrangements. Series chiller plants offer energy and first cost savings, even in VAV systems.
Using multiple-stage centrifugal chillers and putting chillers in series creates higher lift more easily and more efficiently, with a more rigorous, stable operating profile. In contrast, chillers lined up in parallel must each create the coldest water required for the entire system while rejecting heat to the warmest condensing temperature.
While series arrangements of chiller evaporators have been used in many applications,1-3’4-1 series condenser arrangements are less common. A series-counterflow chilled water plant design arranges the evaporators in series, but also arranges the condensers in series using a counterflow configuration. Counterflow means that the condenser water and the evaporator water flow in opposite directions.
Chiller plants with series evaporators and parallel condensers aren’t recognizing the highest efficiency gains because the chiller producing the coldest chilled water must create more lift and therefore do more work. By arranging the condensers in series counterflow, the lift of each compressor is nearly the same. The result is a pair of chillers working together to create high lift while increasing overall plant efficiency.
VARIABLE FLOW
A relatively new concept, the variable-primary system, has removed one barrier to series-chiller plant design. Series chillers often have a higher pressure drop. In traditional primary-secondary systems, constant flow through the chillers equals constant pressure drop and constant pump energy, so the series chillers’ higher pressure drop results in higher pump energy all the time. Varying the flow through series chillers eliminates the penalty for much of the operating hours.
Variable-primary systems send variable amounts of water flow through the chillers to reduce the pumping energy and enable the delta-T seen by the chiller to remain equal to the system delta-T. Because pump energy is approximately proportional to the cube of the flow (subject to losses through the distribution system), even small flow reductions are valuable.
Consider the following ideal relationship between flow and energy in a pumping system:
Pump Energy oc Flow3
For a system using 80% of the design flow—a modest 20% reduction—the energy required is reduced by nearly 50%. In turn, a more aggressive 50% reduction in flow is an 87% reduction in power.
Varying the flow through series chillers reduces the total operating cost, despite an increased pressure drop at design conditions. Improved tube designs and extensive testing in manufacturers’ testing labs have cut minimum water velocities in half, leading to better turndown for all chillers. The additional implicit turndown capability of the series configuration enables further pumping energy savings. Single-pass evaporators and condensers, when practical, reduce water pressure drop and pumping costs compared to two- or three-pass configurations.
VARIABLE FLOW COMBATS THE INCREDIBLE SHRINKING CHILLER
So far, we have only discussed efficiency and energy savings. But what about the installed costs associated with chiller-water system design? Any chiller, big or small, is a significant capital investment. This is one reason most chillers are factory tested for capacity. Cooling capacity is the product of chilled water flow and chilled water delta-T. For the same capacity, as gallons per minute goes up, delta-T goes down.
Chilled water system design has a unique and sometimes dramatic effect on chiller capacity. Primary-secondary chilled water systems shrink chiller capacities, while variable-primary chilled water systems help chiller capacities expand. At times, this chiller capacity expansion can exceed nominal chiller capacity.
Due to this variation in chiller capacity, primary-secondary chilled water systems have excessive chiller starts as well as higher chiller run hours, because more chillers will be operating than necessary. Variable-primary chilled water systems have fewer chiller starts and lower chiller run hours, by squeezing more capacity out of the operating chillers. The plant controller can delay the operation of an additional chiller by increasing the flow through the operating chillers, thus increasing the chiller capacity. Operating the fewest number of chillers possible is a well-known energy optimization strategy.
It is the design of the chilled water system that causes this change in net chiller capacity, not the chiller itself. Chillers are constant flow devices when employed in a primary-secondary chilled water system. A bypass pipe, commonly called a decoupler, serves as a bridge between the primary loop serving only the chillers and the secondary loop distributing chilled water throughout the building to all the air handling units. Because the chiller is a constant flow device, we must subject the chiller to a smaller chilled water delta-T in order to unload the chiller. The decoupler pipe mixes surplus-chilled water with water returning from the load to produce cooler return water for the chiller. But what if some event other than the decoupler lowers the temperature of the returning chilled water? This reduction of chiller-entering water temperature unloads the chiller—even when we don’t want the chiller unloaded.
In a primary-secondary system, additional chillers are sequenced on when the demand for chilled water exceeds the constant flow capacity of each chiller. To illustrate, consider three equally sized chillers in a primary-secondary system. Each chiller is sized for 500 tons and 750 gpm of chilled water (1.5 gpm per ton, or a 16°F chilled water delta-T). When the chilled water distribution system demands more than 750 gpm, two chillers must operate. When the chilled water system demand exceeds 1500 gpm, all three chillers must be on. These chillers will produce their full 500-ton cooling capacity only when the cooling coils create a full 16°F water-temperature rise. If the cooling coils collectively can only produce a 12°F water-temperature rise, the 500-ton chillers can only produce 375 tons. 125 tons of installed cooling capacity is lost. This inability of the chilled water distribution system to achieve the design chilled-water-temperature rise is called “low delta-T syndrome.”
There are several contributors to low delta-T syndrome, but the three greatest offenders are excess distribution pump head, three-way control valves, and chilled water reset. Three-way control valves allow cold chilled water to bypass the cooling coil. The bypassed water is dumped into the return chilled water line, diluting the return chilled water. Any high water-temperature rise created by the cooling coil is destroyed.
The chilled water distribution pump is variable speed, producing more pressure when more chilled water flow is required, and producing less pressure when less chilled water flow is required. The speed of the pump is controlled by sensing the available pressure at the end of the chilled water distribution loop. If this pump creates excess pressure, even two-way control valves will have a difficult time reducing chilled water flow through the cooling coil. When the cooling coil receives excess flow, there is not enough heat in the air to adequately warm the water. Again, water-temperature rise is hampered.
Perhaps the most insidious destroyer of water-temperature rise is chilled water reset. The chiller will consume less energy when it produces warmer chilled water, but chiller energy savings may be dwarfed by the additional pump energy consumption required for delivering warmer chilled water. Coils use less chilled water when it is delivered at a colder temperature. As the chilled water temperature is set upwards, each coil will demand more water to meet the same cooling load.
CHILLER PLANT DESIGN ENERGY COMPARISON
The following example is for a design created initially for the Washington DC Convention Center. Each pair of series-counterflow chillers (assuming multiple-stage compressors on each circuit) has eight to twelve stages of compression equally sharing the load (Fig. 1). The chiller module depicted in Fig. 2 created series-pair efficiencies of 0.445 kW per ton (7.8 COP) at standard ARI rating conditions.
Note: Graph depicts an approximation of actual compressor “lift” based on water temperatures rather than refrigerant temperatures or pressures. Where two or more refrigerant circuits are represented, the evaporators and condensers are piped in a “series-series counterflow” arrangement, respectively.
Fig. 1 Series counterflow chiller arrangement equalizes lift performed by each compressor, minimizing the energy needed to create high lift.
Fig. 3 shows the component and system energy use of various parallel and series chiller configurations using variable evaporator flow with reduced condenser water flow. The series-series counterflow arrangement for the chillers reduces the chiller energy to compensate for additional pump energy. In the case of this particular installation, series-series counterflow saved $1.4 million in lifecycle costs over the parallel-parallel alternative. The low-cost alternative used six electric centrifugal chillers with dual refrigeration circuits, 2 gpm of condenser water per ton of cooling, piped in a “series evaporator-series condenser” arrangement. It also used 1040 kW less than the parallel-parallel configuration.1-5-1
Fig. 2 Module with dual-circuit chillers in series provides 8-12 stages of compression and uses 0.445 kW per ton for the chillers at standard ARI rating conditions.
| Arrangement | Chillers* | Evaporator | Condenser | Cooling Towers | System | ||||||||||
| Evaporator | Condenser | Units/ modules | Compressor
efficiency kW/ton |
Flow gpm | AP Feet of Water | Number
of pumps |
Power per pump kW | Flow gpm | AP Feet of Water | Number
of Pumps |
Power per pump kW | Number of cells | Power per cell kW | Total power kW | Life-cycle cost $USD |
| Parallel | Parallel | 5/5 | 0.649 | 2,800 | 3.26 | 5 | 2.18 | 4,200 | 3.66 | 5 | 3.67 | 8 60 | 7324 | 18,836,302 | |
| Parallel | Parallel | 6/6 | 0.618 | 2,333 | 4.18 | 6 | 2.33 | 3,500 | 3.53 | 6 | 2.95 | 8 60 | 7001 | 18,076,391 | |
| Series | Series-Counterflow (1.5 gpm/ton) | 6/3 | 0.560 | 4,667 | 17.96 | 3 | 19.99 | 5,250 | 14.8 | 3 | 18.54 | 8 48 | 6379 | 16,819,167 | |
| Series | Series-Counterflow (2.0 gpm/ton ) | 6/3 | 0.535 | 4,667 | 17.96 | 3 | 19.99 | 7,000 | 25.2 | 3 | 42.08 | 8 60 | 6284 | 16,656,947 | |
| Series | Parallel (2.0 gpm/ton) | 6/3 | 0.555 | 4,667 | 17.96 | 3 | 19.99 | 3,500 | 3.53 | 6 | 2.95 | 8 60 | 6385 | 16,888,493 | |
| * The chillers represented in this table all have dual refrigerant circuits. The full analysis included single refrigerant circuit chillers at various flow rates and efficiencies. | |||||||||||||||
* The chillers represented in this table all have dual refrigerant circuits. The full analysis included single refrigerant circuit chillers at various flow rates and efficiencies.
Fig. 3 Projected energy-use and lifecycle costs for series and parallel chiller configurations.
DESIGN PARAMETERS
A larger-than-conventional difference between the entering and leaving chilled water temperatures permits a lower flow rate, reducing the initial costs for distributing the chilled water (pumps, piping) in central chilled water plants. Smaller pipes and pumps can then be used to satisfy the same capacity.
Because supplying colder chilled water requires more power from the chillers, the cost savings from reducing the pumping power and pipe size and installation must offset the chiller power increase. Chiller designs and controls have improved to the point at which producing 37°F water no longer causes concern for freezing evaporator tubes. Experience shows that fast, accurate chiller controls and algorithms can safely accommodate temperatures as low as 34°F without the addition of antifreeze.
• Entering-chiller water temperature: 55°F.
• Leaving-chiller water temperature: 37°F.
• Evaporator flow rate/capacity: 1.33gpm/ton.
Many plant configurations are possible:
• Both evaporators and condensers in parallel.
• Evaporators in series and condensers in parallel.
• Both evaporators and condensers in series.
At design conditions:
• Chilled water enters the upstream chiller at 55°F and exits at 45.1°F.
• Chilled water enters the downstream chiller at 45.1°F and exits at 37°F.
• Condenser water enters the downstream chiller at 85°F and exits at 91.3°F.
• Condenser water enters the upstream chiller at 91.3°F and exits at 98.9°F.
FULL-LOAD EFFICIENCY IMPROVEMENT
The series-series counterflow arrangement yields the lowest full-load chiller power (about 14% lower than the parallel-parallel configuration) (Fig. 3). The dramatic reduction in chiller power occurs because the upstream chiller in the series-series counterflow arrangement operates at a higher chilled water temperature, which means that the refrigerant temperature and refrigerant pressure in the evaporator are also higher in the upstream machine. The downstream chiller “sees” a lower con-denser-leaving water temperature—and therefore has a lower condenser refrigerant pressure—than it would in a plant with the chiller condensers arranged in parallel. Fig. 1 illustrates the concept of reduced lift using the design parameters for this chilled water plant. Chiller power can be reduced by decreasing compressor “lift.” In this example, the difference in average lift at design is nearly 13%.budget number, series chiller configuration and low-flow, low-temperature conditions could save $10 million or more in power generation equipment on a 100,000-ton project.
Now, consider a series-series counterflow arrangement of two dual-circuited chillers. Because each of the chillers in this design has two refrigeration circuits, the reduced lift effect is multiplied. Instead of two lifts, there are four. The difference in average lift at design for the system with four independent refrigeration circuits in a series-series counterflow arrangement exceeds 19%.
At the design conditions defined for the system, chiller performance is well above the 6.1 COP requirement set by ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings. At standard ARI rating conditions, each chiller module would operate with an efficiency of 0.445 kW/ton.
PART-LOAD EFFICIENCY IMPROVEMENT
The reduction in lift provided by the series-series counterflow arrangement also occurs at part-load conditions. The temperature of the water leaving the evaporator of the upstream chiller is always warmer than the system water, and the temperature of the water leaving the condenser of the downstream chiller is always cooler than the system water. The upstream chiller does not need to perform the same amount of cooling as the downstream chiller. The benefit comes from the upstream chiller’s ability to produce chilled water at an elevated temperature.
SAVINGS AMPLIFIED BY POWER INFRASTRUCTURE
While the chiller performance is remarkable with this design, the performance conditions for this application were carefully selected to optimize the overall energy consumption of the entire chilled water plant. Series chiller configurations are not just about the chiller or the pump savings, but reduced electrical infrastructure requirements and environmental impact. The previous example was a 10,500-ton plant. Consider the reduction in power generation requirements when multiplied tenfold. In a 100,000-ton cooling and power infrastructure project, increasingly common in the Middle East and China, our example’s 1040-kW reduction blossoms into nearly 10 MW. A conservative estimate for the cost of the generation equipment is $1000 per kW. Using that round
PACKAGED SOLUTIONS MINIMIZE COMPLEXITY AND RISK
Frequent users of these concepts are packaged chiller plant manufacturers that are pre-engineering and packaging series-counterflow chillers with built-in optimization controls. Packaged chiller plants utilizing series-counterflow chiller arrangements are currently available in sizes up to 8000 tons. The chillers in these packaged chiller plants can be factory-performance tested in accordance with ARI procedures prior to shipment from the chiller manufacturers’ facility.
Packaging companies and astute engineers have put two series-counterflow chillers in series with each other— essentially creating a 4-chiller series module (Fig. 2). These solutions enhance the thermodynamic benefit created by series-counterflow chillers while minimizing complexity and risk for the system owner and operator.
CONTROL STRATEGIES
How the plant should respond to varying system conditions is a topic for discussion with the design engineer, plant owner, and plant operators. For example, if the entering-chiller water temperature is not reaching design conditions, the operators could:
1. Increase pump speed or turn on more pumps to increase flow rates and more fully load the active chillers.
2. Reset the setpoints of the upstream chillers to 55% of the total temperature difference. Lowering the setpoint of the upstream chillers as the result of a drop in entering-chiller water temperature lessens the benefit of reduced lift. However, the upstream chillers will always run at a higher evaporator pressure than the downstream chillers, which saves energy consumption and costs.
3. Address chiller sequencing in the context of the system options, variable or constant chiller flow, extra pumps, orotherconcerns. Itmightbe mostcost effective to use a startup strategy that fully loads one chiller module and then activates the remaining chillers in modules (pairs). Activating the upstream chiller and operating it at the higher water temperature takes advantage of all of the available heat transfer surface area without increasing the energy consumed by ancillary equipment.
USEFUL REDUNDANCY
Large chiller plants can be more adaptive and efficient with multiple chillers rather than fewer large, field-erected chillers. In plants with more chillers, redundancy is easily created through parallel banks of upstream and downstream chillers. Different combinations of upstream and downstream chillers can meet the load, so if one chiller is being serviced, its duty can be spread out to the other chillers. The same is true for pumps, which do not have to be sequenced with the chillers.
EFFICIENCY, FLEXIBILITY IN SMALLER PLANTS AND RETROFITS
The benefits of low flow, low temperature, and high efficiency apply to other types of chillers as well. Smaller, noncentrifugal chillers can benefit proportionately more under these conditions when placed in series.
Helical-rotary chillers are sensitive to increased lift and decreased condenser water flow. Absorption chillers struggle to make water colder than 40°F, and their cooling capacity increases when placed upstream. Both can be put upstream in the sidestream position for reduced first cost and higher efficiency. Reusing existing, older, less efficient chillers upstream is also an interesting option to explore. These sidestream configurations combine the benefits of series and parallel chillers while isolating some chillers from water flow variations.
CONCLUSION
As chiller efficiencies continue to improve, district energy and central plant designers can optimize the entire system to achieve even lower costs of ownership. Owners can expect more first cost and energy savings from low-flow, low-temperature and highly efficient chiller configurations. The unique benefits and flexibility of series chiller plant designs with variable-primary pumping arrangements include lower overall chilled water system operating costs, reduced emissions, and improved environmental responsibility.
