Publication: HVAC & R Research
Publication Date: 01-JUL-07
Delivery: Immediate Online Access
Author: Braun, James E.
Article Excerpt
Received September 13,2006; accepted February 23, 2007
INTRODUCTION
A chiller plant often consists of multiple chillers, multiple condenser water pumps, and multiple cooling towers as depicted in Figure 1. In some situations, it is economical to employ a mix of chillers that are "powered" by electricity and natural gas (e.g., absorption or engine-driven). This is termed a hybrid chiller plant. The major advantage of using natural gas chillers in hybrid central chiller plants is a reduction in peak electrical demand and on-peak energy usage, which can reduce overall operating costs. The electric demand cost can often account for about half of the total air-conditioning bill.
[FIGURE 1 OMITTED]
Proper control of cooling tower fans in cooling plants can have a significant impact on operating costs. For all electric plants, optimal control of cooling fans has been shown to result in significant (e.g, 5%--15%) savings in plant energy costs as compared with typical strategies that are employed (see Sud [1984], Lau et al. [1985], Hackner et al. [1985], Klein et al. [1988], Braun [1988], Braun et al. [1989a, 1989b], ASHRAE [2003]). The savings for plants employing absorption chillers can be even larger because of higher heat rejection requirements. For instance, Koeppel et al. (1995) simulated optimal control of tower fans and condenser pumps for a cooling plant having a double-effect absorption chiller and determined a 20% reduction in costs compared to using fixed speeds with a tower bypass control to maintain a constant cooling tower water supply.
Although there is a large body of literature related to supervisory control for all-electric plants, there is very little literature on supervisory control of absorption, engine-driven, and hybrid chiller plants. Braun and Diderrich (1990) developed an algorithm for cooling tower fan control for all-electric plants that is included in the 2003 ASHRAE Handbook--HVAC Applications (ASHEAE 2003). However, this strategy is not appropriate fro hybrid plants because it is based on minimizing input energy usage rather than cost.
Koeppel et al. (1995) developed a simplified strategy for cooling tower fan control for absorption cooling plants that involves the determination of a linear relationship between a setpoint for cooling tower supply water temperature and ambient wet-bulb temperature. Optimization results were used to determine a simple linear model for a case study involving a single double-effect absorption chiller. The application of the simple strategy resulted in savings that were nearly identical to the optimization results. However, it's not obvious how linear control relationships would be determined in practice and how to apply this method to hybrid plants.
This paper develops a general algorithm for control of cooling tower fans for cooling plants that have any combination of electric and natural-gas chillers. The development follows an approach that is similar to the development of Braun and Diderrich (1990). The control method is evaluated through comparisons with optimal control for a range of different cooling plants and operating conditions using a simulation tool. Optimal control means that the control is based on minimization of an operating cost function that incorporates perfect information about the plant. A nonlinear optimization was performed to determine the performance for the benchmark optimal control using the simulation tool. The development of the near-optimal control algorithm used several simplifying assumptions and heuristics in order to determine analytical expressions for cooling tower control. In this context, near-optimal control implies that the strategy has performance that is close to that associated with optimal control.
DEVELOPMENT OF A NEAR-OPTIMAL COOLING TOWER CONTROL ALGORITHM
Chiller plants, such as that depicted in Figure 1, typically have multiple cooling towers with fans that have multiple speeds of operation. In general, optimal control of cooling tower fansresults from a trade-off in the cost of operating the chillers and cooling tower fans. The energy consumption of a chiller is sensitive to the condenser water temperature, which is affected by the cooling tower control. Increasing the tower airflow reduces the chiller energy requirement but at the expense of an increase in fan power consumption. For a given set of conditions, an optimal tower control exists that minimizes the sum of the chiller and cooling tower fan power.
Braun and Diderrich (1990) described how the determination of optimal tower fan control can be separated into two parts: tower sequencing and optimal airflow. For a given total tower airflow, optimal tower sequencing specifies the number of operating cells and the fan speeds that give the minimum fan power consumption. Once the tower sequencing is specified, then the optimal airflow can be determined by analyzing the trade-offs between the costs of operating the chiller and the fan.
This section presents the development of an algorithm for near-optimal control of cooling towers that is based upon a combination of heuristic rules for tower sequencing and an open-loop control equation derived from a detailed analysis.
Optimal Fan Sequencing
Simple relationships exist for the best sequencing of cooling tower fans for towers having multiple cells as capacity is added or removed. When additional tower capacity is required, Braun et al. (1989a, 1989b) have shown that in almost all practical cases, the speed of the tower fan operating at the lowest speed (including fans that are off) should be increased first. Similarly, for removing tower capacity, the highest fan speeds are the first to be reduced. This leads to the following general rules for sequencing of tower fans:
1. All Variable-Speed Fans: Operate all cells with fans at equal speeds.
2. Multi-Speed Fans: Increment lowest-speed fans first when adding tower capacity. Reverse for removing capacity.
3. Variable/Multi-Speed Fans: Operate all cells with variable-speed fans at equal speeds. Increment lowest-speed fans first when adding tower capacity with multi-speed fans. Add multi-speed fan capacity when variable-speed fan speeds match the fan speed associated with the next multi-speed fan increment to be added.
Criteria for Optimal Tower Airflow
Most cooling towers utilize single-or two-speed fans, such that the optimization problem is discrete rather than continuous. However, for the purpose of estimating the control parameters, it is sufficient to consider the flow as being continuously adjustable. Consider the problem of determining the optimal tower control for continuously adjustable tower airflow. The minimum combined chiller and tower fan cost occurs at a point where the rate of change of...
NOTE: All illustrations and photos have been removed from this article.
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