Cooling/Heating Central Plant Optimization
Cooling and heating central plants offer several opportunities to reduce energy operating costs through optimal or near-optimal controls for individual equipment (local optimization) and for the entire HVAC system (global optimization). Although optimal controls have been developed and implemented for various components of cooling and heating central plants, global optimization remains a considerably complex endeavor and only a few strategies have been suggested and tested.
In this section, some of the local optimal control strategies are discussed. In addition, operating strategies for entire cooling/heating plants are briefly discussed.
Single Chiller Control Improvement
Before replacing an existing chiller, it may be more cost-effective to consider other cooling alternatives or simple operating strategies to improve cooling plant energy performance. In particular, a significant improvement in the overall efficiency of a chiller can be obtained through the use of automatic controls to:
- • Supply chilled water at the highest temperature that meets the cooling load
- • Decrease the condenser water supply temperature (for water-cooled condensers) when the outside air wet-bulb temperature is reduced
Example 10.3 illustrates typical energy cost savings due to improved controls for a single chiller cooling plant.
Example 10.3
A centrifugal chiller (having a capacity of 500 kW and an average seasonal COP of 4.0) operates with a leaving chilled water temperature of 4.5°C. Determine the cost savings incurred by installing an automatic controller that allows the leaving chilled water temperature to be set 2.5°C higher on average. Assume that the number of equivalent full-load hours for the chiller is 1,500 per year and that the electricity cost is $0.07/kWh.
SOLUTION
Using Figure 9.13 (refer to Chapter 9), the increase in the COP for a centrifugal chiller clue to increase in the leaving chilled water temperature from 4.5°C to 7.0°C is about 8 percent. The energy use savings can be calculated using Eq. (9.12) with SEER,, = 4.0; SEER, = 4.0 x 1.08 = 4.32; NhC = 1,500; Qc = 500 kW; and LFC = 1.0 (assume that the chiller is sized correctly):
Therefore, the annual energy cost savings are $970/year.
Controls for Multiple Chillers
When a central cooling plant consists of several chillers, a number of control alternatives exist to meet a building’s cooling load. Effective controls would select the best alternative for operating and sequencing the chillers to minimize the cooling plant operating costs.
Simple guidelines can be followed to operate multiple chillers at near-optimal performance. Typically, chiller operating variables such as chilled water temperature and condenser water flow rate are adjusted to ensure optimal controls. Some of the near-optimal control guidelines to operate electrically driven central chilled water systems are summarized below (ASHRAE. 2007):
- • Multiple chillers should be controlled to supply identical chilled water temperatures.
- • For identical chillers, the condenser water flow rates should be controlled to provide identical leaving condenser water temperatures.
• For chillers with different capacities but similar part-load performance, each chiller should be loaded at the same load fraction. The load fraction for a given chiller can be set as the ratio of its capacity to the sum total capacity of all operating chillers.
To determine the optimal chiller sequencing, a detailed analysis is generally needed to account for several factors, including the capacity and the part-load performance of each chiller and the energy use associated with all power-consuming devices such as distribution pumps. Chapter 9 provides some guidelines on the potential energy savings associated with multichiller cooling plants.
Controls for Multiple Boilers
As discussed in Chapter 8, the use of an array of small modular boilers provides a more energy-efficient heating system than a single large boiler, especially under part-load operation conditions. Indeed, each of the modular boilers can be operated close to its peak capacity, and thus its highest energy efficiency. To optimally operate multiple boilers, it is important to know when to change the number of boilers online or offline. The mere addition of a second boiler online when one boiler cannot handle the load may not provide the minimum operating cost. Indeed, the increase of firing rate (due to additional heating load) on any given boiler can cause a decrease in thermal efficiency due to higher flue-gas temperatures, and thus higher thermal losses. However, the addition of a second boiler online increases the standing losses due to auxiliaries and the thermal losses through the added casing and piping of the second boiler. Therefore, a detailed analysis is needed to determine the changeover points for the multiple boilers. These changeover points depend on the characteristics of each boiler (ASHRAE. 2007).
SUMMARY
In this chapter, an overview of basic components and applications of HVAC control systems has been presented. In particular, the energy cost savings incurred by various functions of EMCS have been illustrated through selected examples and applications. In addition to being knowledgeable regarding the currently available control systems and applications, the energy auditor should be aware of the development of intelligent control systems, especially those applicable to HVAC systems.
PROBLEMS
10.1 A hot water heating coil has a set-point of 110°F with a throttling range of 15°F. The heat output of the coil varies from 0 kW to 80 kW. Assuming that a proportional controller is used to maintain the air temperature set-point, determine the proportional gain for the controller and the relationship between the output air temperature and the heat rate provided by the coil. Assume steady-state operation.
- 10.2 A chiller water heating coil has a set-point of 13°C, with a throttling range of 3°C. The cooling capacity of the coil varies from 0 kW to 20 kW. Assuming that a proportional controller is used to maintain the air temperature set-point, determine the proportional gain for the controller and the relationship between the output air temperature and the cooling rate provided by the coil.
- 10.3 Determine the reduction in the annual energy costs due to staggered duty cycling of six identical fan motors, each rated at 20 hp and has 4 poles. The motor manufacturer specifies 20 minutes on and 10 minutes off as the minimum duty cycle. The utility monthly demand charge is $15/kW.
- 10.4 Repeat Problem 10.3 but with motors with (a) 2 poles and (b) 6 poles. Comment on the impact of the number of poles.
- 10.5 Determine the annual electricity savings when the condensing temperature is reduced on average by 5°C by an automatic controller for a 500-ton reciprocating chiller with a seasonal COP of 3.8. Assume that the number of equivalent full-load hours for the chiller is 2,500 per year and that the electricity cost is $0.10/kWh.
- 10.6 A 300-ton screw chiller with an efficiency of 0.70 kW/ton operates with a leaving chilled water temperature of 4.5°C. Determine the cost savings incurred by installing an automatic controller that allows the leaving chilled water temperature at 7.5°C higher on average. Assume that the number of equivalent full-load hours for the chiller is 3,500 per year and that the electricity cost is $0.12/kWh.
