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Pre-cooling Chilled Water Return--Replacing Yesterday's Strainer Cycle.

INTRODUCTION

An opportunity for energy savings exists where chilled-water cooling systems are not required to have economizers. For example, chilled water fan coil units (FCUs) serving guest rooms in hotels are not usually required to have economizers as each individual fan unit typically falls below the cooling capacity threshold in ANSI/ASHRAE/IESNA Standard 90.1(ASHRAE 2010). While it is not typically cost-effective for fan units smaller than 54,000 to have airside economizers (PECI and Taylor Engineering 2011), when there are many small units served by a common chilled water system, a water economizer can provide lower cost cooling and can be cost effective when there is a large enough total capacity. This proposal would also expand the benefit of economizer cooling to non-fan units served by chilled water such as radiant cooling and both active and passive chilled beams.

WATER ECONOMIZERS

After the energy crises of the 1970's, many building operators reduced cooling loads by using a "strainer cycle" that used cooling tower water directly in the chilled water system with a strainer in place to remove particulate contaminants from the cooling tower water. This practice was abandoned due to chilled water system fouling, and a heat exchanger was introduced to separate tower water from the chilled water system while achieving similar energy benefits.

A water economizer is a system by which the supply air of a cooling system or chilled water in a non-fan cooling system is cooled indirectly with water that is itself cooled by heat or mass transfer to the environment without the use of mechanical cooling (ASHRAE 2010). There are several system types that can benefit from water economizers where air economizers are not practical or cost effective. Such systems include radiant cooling, chilled beams, computer room air conditioners, and fan coil units. Water economizers can be configured in a number of ways. A separate evaporative fluid cooler (closed-circuit cooling tower) can be used to pre-cool chilled water return in a typical primary/secondary chilled water system as shown in Figure 1. The necessary additional components to create the water economizer benefits are shaded in blue.

Using a separate fluid cooler allows maximum water economizer effect, as the economizer tower is not impacted by the need to maintain a minimum condenser entering water temperature when the chiller is operating. This allows the best integrated operation and enhances system stability as transition from water-economizer only to integrated operation is seamless. This arrangement also allows only the heat exchanger and primary chilled water pumps to be used when the chiller is not needed. Further, this arrangement can be applied to either water-cooled chiller systems, air-cooled chiller systems, or to buildings served by a district cooling plant. An additional advantage of this system is that it can provide partial cooling while the chiller maintains the chilled water supply (CHWS) temperature necessary for dehumidification where required (integrated water economizer). With its decoupled hydronic attachment to the chilled water system, the closed-circuit cooling tower chilled water return (CHWR) pre-cooler would have a similar impact on the chilled water plant as a reduction in general cooling load. On larger systems, there may be some benefit for the constant volume pump to the fluid cooler to be variable flow.

PROVISIONS FOR COLDER CLIMATES

In colder climate zones (CZ), CZ 5 and colder, a flat plate heat exchanger and pump are added to the warm climate primary/secondary pumping configuration from Figure 1, as shown in Figure 2. This allows operation in freezing conditions, although there is a reduction in savings due to the needed approach at the extra heat exchanger and additional pumping energy. This arrangement allows only the heat exchanger pumps and main supply pumps to be used when the chiller is not needed. An extra heat exchanger allows operation in freezing conditions by using glycol in the final loop that is exposed to the weather. When there is danger of freezing, the fluid cooler is drained and operated as a dry cooler. Other approaches, beyond the scope of this paper, can be applied to tower freeze protection (Hanson 2013).

COST EFFECTIVENESS

To test whether the approach is cost effective, analysis of an evaporative closed-circuit pre-cooler for return chilled water is applied to loads for a cooling system without outside air economizers. Analyzing this system provided the best indication of the cost effectiveness of a water economizer approach, as it clearly showed the savings from adding a discrete water economizer that was sized to match full cooling load without a chiller at 50[degrees]F DB and 45[degrees]F WB (10[degrees]C DB and 7[degrees]C WB) outside air conditions, matching the ASHRAE Standard 90.1 water economizer sizing requirements. As an independent pre-cooler add on, the control strategy was simple and there were few interdependencies with the main system that would complicate either the savings or cost analysis. The cost-effectiveness analysis found that the approach described above was cost effective in all climate zones except CZ 1A, represented by Miami, Florida.

Energy Impact

Based on average national energy price (1) of $0.1032 per kWh, the energy savings were determined based on post processing simulated loads from a large hotel modeled in EnergyPlus[TM]. (2) Guest rooms served by fan coil units produced the cooling loads that were not served by an air economizer. As a proxy for cooling loads, guest rooms have relatively low internal loading, so cooling savings are likely to be higher in other occupancies.

The electric savings from precooling the CHWR with a water economizer are shown in Figure 3. The gross chiller savings are shown, and added energy for closed-circuit cooling tower pumps and fans are subtracted resulting in a net energy savings. The selected climate zones analyzed are shown. Based on this analysis, there is demonstrated net savings from using a water economizer in all the analyzed climate zones.

Basis for Savings Analysis

For the climate zones analyzed, the savings from using a system addition shown in Figure 1 or 2 (depending on climate zone) was developed. Then the life cycle operating cost was found, using the prototype loads, typical cooling tower and pump performance criteria, and life cycle cost parameters. For each location, the incremental energy cost savings was found, arriving at a net present value of energy savings for the proposed water economizer addition.

Pacific Northwest National Labs (PNNL) analyzed the savings for the water economizer based on hourly cooling loads for systems without air economizers. The hourly cooling loads were modeled in EnergyPlus with the Large Hotel prototype used in Standard 90.1 evaluation. This prototype is one of 16 prototype building energy models developed by PNNL to provide analytical support for the Standing Standard Project Committee (SSPC) 90.1(Thornton et al. 2011). The Large Hotel prototype includes both guest rooms served by fan coil units without air economizers and larger lobby and meeting room systems with air economizers. In the analysis, the peak cooling loads served by the water economizer ranged from 50 tons to 80 tons, depending on climate zone. While in the model these zones were served by fan coil units, the load would be similar if served by radiant cooling systems or chilled beams. The chillers simulated in the EnergyPlus model meet minimum efficiency under ASHRAE Standard 90.1-2010 (ASHRAE 2010).

Coil load response regressions for variable flow vs. load were developed from a typical fan-coil unit cooling coil to determine CHWR based on CHWS and load. A closed-circuit cooling tower regression was developed from manufacturer performance data to determine the tower leaving temperature based on outside wet-bulb temperature and the CHWR entering the tower. For each climate zone analyzed, the cooling load without an air economizer was determined and a closed-circuit cooling tower and associated fans and pumps were sized to meet the Standard 90.1 water economizer requirement that full cooling load be met at 50[degrees]F DB and 45[degrees]F WB (10[degrees]C DB and 7[degrees]C WB) outside air conditions.

A spreadsheet was used to evaluate the impact of the water economizer on the hourly chiller loads from the prototype model. The hourly chiller savings were based on load offset by the water economizer and the chiller COP from the hourly EnergyPlus model. Net savings were determined by subtracting the hourly energy use of the fluid cooler spray pump, chilled water pre-cool pumps, and the fluid cooler fan energy from the hourly chiller savings. Fluid cooler spray and chilled water pre-cool pumps were assumed to be constant flow. The fluid cooler fans simulated are large enough to require variable speed, so energy was adjusted based on the ratio of the potential full airflow rejected heat to the actual cooling load.

Basis for Costs

Incremental costs were developed for all equipment shaded in blue in Figures 1 and 2 and other required system components. This included a separate fluid cooler, a heat exchanger with glycol in cold climates, pumps, electric starters and connections, controls, piping, fittings, and valves. Components were sized to meet the fan coil unit cooling load at 50[degrees]F DB and 45[degrees]F WB (10[degrees]C DB and 7[degrees]C WB) outside air conditions, to comply with the standard 90.1 water economizer requirement. Engineering cost estimates for the water economizer systems are shown in Table 2. Costs ranged from $34,000 to $44,000 in hot and moderate climates with water economizer capacities from 19 to 30 tons (67 to 105 kW).

In cold climates (CZ 5 through 8), due to the higher cost and greater temperature approach introduced by the extra heat exchanger and other freeze protection components, the system was not cost effective when sized to meet the load in the Large Hotel prototype. Larger systems achieve an economy of scale, as the incremental cost of adding heat rejection capacity is less than the cost per ton for a smaller system. A sensitivity analysis was performed for the cold climates to determine how large a cooling load was needed for cost effectiveness. Based on the analysis, a size approximately double the warm climate system size was found appropriate. For cold climates, costs ranged from $78,000 to $102,000 and the added fluid coolers had a nominal capacity in the 55 to 78 ton (193 to 275 kW) range. While discrete thresholds could be established for each climate zone, the approach was to simplify the proposal by grouping climate zones based on the need for freeze protection.

A maintenance cost allowance was included to cover the expected cost of water, chemical treatment, and added tower and pump maintenance. While a detailed maintenance analysis was not conducted for each climate zone, case studies show cooling tower annual maintenance at $12 to $25 per ton (ASHRAE 2011). To be conservative, the higher number of $25 per ton was used, adjusted for the size of the added tower in each analyzed climate zone.

Life-Cycle Cost

Two different cost effectiveness methods were applied:

* The ASHRAE 90.1 committee scalar method uses economic factors to arrive at a discounted threshold or target simple payback period (SPP) based on the expected life of each energy conservation measure. If the calculated simple payback is less than this target, it is deemed to be cost effective. This method accounts for tax impacts and uses a nominal discount rate appropriate for private commercial property owners.

* The DOE/FEMP method uses an institutionally oriented discount rate, fuel escalation costs, and the expected life to determine the present value of savings for a particular measure. When divided by the cost, the result is a benefit to cost ratio (BCR). When that BCR is greater than one, a measure is considered cost effective. This method does not include tax considerations or the opportunity value of invested capital.

Economic factors for the scalar method are those arrived at by the ASHRAE 90.1 committee for analysis of Standard 90.1-2013 measures. National average electric and gas rates are from EIA for 2011. The DOE/FEMP discount rate and electric and gas Uniform Present Worth Factors (UPWF) are from the NIST Life Cycle Cost 2011 supplement (Rushing et al. 2011). The factors shown in Table 1 are used.

The cost effectiveness results are shown in Table 2 along with the proposed requirement thresholds. The ASHRAE simple payback period (SPP) is compared to a discounted payback limit or scalar of 13.077 years. The simple payback period (SPP) is the cost of the project divided by the annual cost savings in dollars (annual energy savings minus maintenance cost). The discounted payback limit (also known as the scalar) is calculated using a method and agreed to parameters developed by the ASHRAE 90.1 standard committee (McBride 1995). The discounted payback limit accounts for discounting, tax impacts, and fuel escalation and a measure is cost effective when the SPP is less than the discounted payback limit. Cost effectiveness under the FEMP criteria is shown when the BCR is greater than 1.0. The fluid cooler size to meet the water economizer requirement is shown, as is the design cooling capacity for the systems that do not have air economizers (Non-Air Design Cooling Tons/kW).

Under both analysis methods, the added water economizer is cost effective in all analyzed climate zones except 1A (Miami). The analyzed climate zones were selected to cover the range of moisture level and temperature extremes. In dry climate zones, the measure is cost effective in both hot and cold extremes (1B & 8). In moist climate zones, it is cost effective in both 2A and 3A, but not climate zone 1A. It is also cost effective in intermediate climate zones 4C, 4A and 5A. By covering the extreme conditions, and both moist climate zones neighboring 1A where it was not cost effective, the analysis indicates that the measure will be cost effective in all climate zones except 1A. This analysis is consistent with earlier exceptions to ASHEAE 90.1 that excluded climate zone 1A from an economizer requirement. Since the water economizer approach is quite cost effective in climate zone 1B, it makes sense to require it for chilled water systems in climate zone 1B. Therefore, the additional water economizer is recommended for chilled water systems in all climate zones except 1A for chilled water systems that do not have an airside economizer.

STANDARD 90.1 REQUIREMENTS AND PROPOSED ADDENDUM DU

Standard 90.1-2010 requires air or water economizers for each cooling system that has a fan and a cooling capacity greater than 54,000 Btu/hr. There are a number of exceptions, including systems in climate zones 1A and 1B. This analysis demonstrates that the cost effectiveness of a water economizer should not be determined by the individual fan cooling unit size, but rather by the total capacity of units without an air economizer served by a chilled water system, regardless of whether or not they include a fan. In addition, it demonstrates that unlike an air economizer, the water economizer is cost effective in climate zone 1B due to its low cooling design wet-bulb temperature.

Addendum DU is proposed to extend the economizer requirements in Standard 90.1 to chilled-water cooling systems with or without a fan, in all climate zones except 1A, where the cooling capacity of chilled-water systems without air economizers exceeds thresholds determined to be cost effective. These thresholds--shown in Table 2--were determined based on the cost-effective system size in each climate zone for a separate closed-circuit tower and accessories. Addendum DU includes higher thresholds for chilled water systems served by air-cooled chillers or district chilled water plants to allow for the added cost of maintaining a closed-circuit cooling tower in systems that otherwise would not have one.

MULTIPLE OPTIONS

For this analysis, a relatively straightforward system addition to a traditional constant-speed chiller was selected. Other approaches can be used that may reduce the cost of a water economizer. An alternative method would use the existing cooling towers with a plate-and-frame heat exchanger for the water economizer. In this approach, the sequence moves through three phases:

* A total economizer phase where the chiller is off, and total cooling load is met by evaporatively cooling the chilled water return.

* A mixed phase where both the condenser water and chilled water return are cooled by the same tower.

* Operation with only the chiller when there is no benefit to chilled water return pre-cooling.

This approach can save cost, but the savings are more dependent on the mix of non-economizer and air economizer loads served by the cooling plant, and the ability to pre-cool chilled water return may be limited by the need to maintain a high condenser water supply (CWS) temperature to satisfy the minimum lift requirements set by the chiller manufacturer. In some cases and locations the savings can be greater than those shown in Figure 3, especially where a larger cooling tower can extend the time that cooling can be provided without chiller operation or a larger overall cooling load can benefit from precooling. A schematic with one possible configuration for a mixed approach is shown in Figure 4.

Using chillers designed for low lift can allow colder than normal condenser water to be used and result in increased savings (Stein and Taylor 2013). Another options that may reduce costs include isolating one of the normal plant cooling towers for pre-cooling until it is needed for peak capacity. Either of the systems shown in Figures 1, 2 or 4 or multiple hybrid or alternate systems can be used to meet the proposed requirement as long as the cooling load for systems without air economizers can be met solely with the water economizer and without a chiller at 50[degrees]F DB and 45[degrees]F WB (10[degrees]C DB and 7[degrees]C WB) outside air conditions and as long as integrated operation with both the water economizer and chiller is allowed.

CONCLUSION

Pre-cooling chilled water return with an evaporative fluid cooler for systems that do not have airside economizers meets the ASHRAE 90.1 and FEMP cost effectiveness criteria in all climate zones except 1A, when applied to a chilled water load above certain capacity thresholds. Several arrangements that may reduce cost can be used, but the cost effectiveness criteria were met even when modeled using an arrangement that includes a separate fluid cooler. Given an increased capacity threshold to allow for increased maintenance, the approach is also cost effective for air-cooled chillers and buildings on a district cooling service.

REFERENCES

ASHRAE. (2010). ASHRAE/ASHRAE/IES Standard 90.1-2010: Energy Standard for Buildings except Low-Rise Residential Buildings. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA.

ASHRAE. (2011). 2011 ASHRAE Handbook Applications. American Society of Heating, Refrigerating and Air Conditioning Engineers [ASHRAE], Atlanta, GA.

Hanson, S. (2013). "Water-Side Economizer Design and Modeling." ASHRAE 2013 Winter Meeting. Denver, Colo.

McBhde, M. F. (1995). 'Development of Economic Scalar Ratios for ASHRAE Standard 90.1 R." Proceedings of Thermal Performance of the Exterior Envelopes of Buildings VI, ASHRAE, ASHRAE.

PECI, and Taylor Engineering. (2011). "Light Commercial Unitary HVAC, 2013 California Building Energy Efficiency Standards, CODES AND STANDARDS ENHANCEMENT INITIATIVE (CASE)." California Utilities Statewide Codes and Standards Team for California Energy Commission.

Rushing, A., Kneifel, J., and Lippiatt, B. (2011). "Energy Price Indices and Discount Factors for Life-Cycle Cost Analysis -- 2011." NIST for USDOE FEMP.

Stem, J., and Taylor, S. (2013). "Integrated Water-Side Economizer." ASHRAE 2013 Winter Meeting. Denver, Colo.

Thornton, B. A, Wang, W., Cho, H, Xie, Y., Mendon, V. V., Richman, E. E., Zhang, J., Athalye, R A., Rosenberg, M. I., and Liu, B. (2011). "Achieving the 30% Goal: Energy and Cost Saving Analysis of ASHRAE/IES Standard 90.1-2010." PNNL for USDOE.

Reid Hart, PE

Member ASHRAE

Jeff Boldt, PE

Member ASHRAE

Michael Rosenberg

Member ASHRAE

Reid Hart, PE, Senior Building R&D Engineer and Michael Rosenberg, Senior R&D Scientist, both with Pacific Northwest National Laboratory (PNNL) in Richland, Wash. Jeff Boldt, PE, Principal - Director of Engineering with KJWW Engineering in Madison, Wis.

(1) Weighted commercial national average energy prices developed by the ASHRAE 90.1 standards committee for analysis of 90.1-2013 proposals; based on national data from the 2011 Annual Energy Outlook by US Energy Information Administration. www.eia.gov.

(2) EnergyPlus is an advanced building simulation program. More information online at: http://apps1.eere.energy.gov/buildings/energyplus/
Table 1. Economic Factors

Economic Parameter                ASHRAE SPP Scalar Method

Economic Life - Years                     22
Fuel Escalation Rate - %                   3.76%
Electnc UPWF                                N/A
Maintenance UPWF                            N/A
Discount Rate - %                          7.00%
Loan Interest Rate - %                     6.25%
Federal Tax Rate - %                      34.00%
State Tax Rate - %                         6.50%
Heating - Gas Price - $/therm             $0.9900
Cooling - Electric Price - $/kWh          $0.1032
Metric for cost effectiveness               SPP
Metric threshold                        < 13.077 years

Economic Parameter                          DOE/FEMP NPV

Economic Life - Years                           22
Fuel Escalation Rate - %          (varies throughout study period)
Electnc UPWF                                    15.31
Maintenance UPWF                                15.94
Discount Rate - %                                3.00%
Loan Interest Rate - %                           N/A
Federal Tax Rate - %                             N/A
State Tax Rate - %                               N/A
Heating - Gas Price - $/therm                   $0.9900
Cooling - Electric Price - $/kWh                $0.1032
Metric for cost effectiveness                    BCR
Metric threshold                               > 1

Table 2. Cost Effectiveness Results

City       CZ  Net KWh   Save/yr @    Estimated    ASHRAE
                saved   $0.1032 /kWh    Cost     Scalar SPP

Miami      1A   14,800     $1,527      $34,000     22.26
Riyadh     1B   92,100     $9,505      $44,000      4.63
Houston    2A   48,200     $4,974      $42,000      8.44
CZ 1B-2        Requirement Threshold: 960,000
                Btu/hr Cooling Design (280 kW)
Memphis    3A   61,000     $6,295      $42,000      6.67
Baltimore  4A   67,900     $7,007      $42,000      5.99
Salem      4C   72,300     $7,461      $40,000      5.36
CZ 3-4         Requirement Threshold: 720,000
               Btu/hr Cooling Design (210 kW)
Chicago    5A   77,200     $7,967      $78,000      9.79
Fairbanks  8    82,000     $8,462      $102,000    12.05
C Z5-8         Requirement Threshold: 1,320,000
                 Btu/hr Cooling Design (385 kW)

City       FEMP    Fluid Cooler             Non-Air Design
            BCR  Sizing Tons (kW)          Cooling Tons (kW)

Miami      0.69       19 (67)                   66 (232)
Riyadh     3.31       30 (105)                  79 (278)
Houston    1.81       27 (95)                   76 (267)
CZ 1B-2    Requirement Threshold: 960,000
                                             80 tons (281)
Memphis    2.29       28 (100)                  56 (197)
Baltimore  2.55       28 (100)                  59 (207)
Salem      2.86       25 (90)                   55 (193)
CZ 3-4     Requirement Threshold: 720,000
           Btu/hr Cooling Design (210 kW)    60 tons (211)
Chicago    1.56       55 (195)                 110 (387)
Fairbanks  1.27       78 (275)                 100 (352)
C Z5-8     Requirement Threshold: 1,320,000
             Btu/hr Cooling Design (385 kW)  110 tons (387)
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Author:Hart, Reid; Boldt, Jeff; Rosenberg, Michael
Publication:ASHRAE Conference Papers
Date:Dec 22, 2014
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