Playing the Smart Hand

Hybrid central plants have proven to offer many advantages, including operational reliability and risk mitigation. But the primary purpose of choosing multiple energy sources is economics: depending on crossover utility rate opportunities, it can reward the forward-looking owner by offering the flexibility to transfer loads during peak periods.


Hybrid central plants have proven to offer many advantages, including operational reliability and risk mitigation. But the primary purpose of choosing multiple energy sources is economics: depending on crossover utility rate opportunities, it can reward the forward-looking owner by offering the flexibility to transfer loads during peak periods.

Many proactive owners have already seen the logic of supplementing their electric chillers with either gas-fired absorption two-stage or gas-engine-driven chillers. In the past, design engineers involved in the creation of these hybrid plants have prescribed sequences of staging chillers based on stable utility rates. But substantial fluctuations in both gas and electric prices, most notably in California, call for new methods—specifically, the consideration of a wider range of determining factors, including dynamic utility rates.

Revisiting a past project

The changing utility market does not only require new methods for future hybrid plants—many of those same forward-looking owners from years past are being forced to reevaluate the controls that were set in place under outdated, stable circumstances.

For example, several years ago we worked with a client that wanted to add redundancy to one of their 2,000-ton central plants located in the California desert. Experiencing loads ranging from 70% to 100% of peak for more than half of the summer months, the original plant operated on two 1,000-ton electric centrifugal chillers.

At the time, the recommendation was made to add two 500-ton direct-fired absorption chillers; these natural-gas chillers could produce each ton-hour at a lower cost during the peak electrical price periods. In addition, energy simulations were performed utilizing the relatively stable electric and gas rates. The results indicated it was only cost-effective to operate the gas chillers as the lead during the higher summer on-peak period. Based on this analysis, the prescribed sequence of operation for staging the chillers was set in place.

However, with the recent fluctuations in utility prices, the company requested a follow-up analysis. The approach, in this case, involved using varying gas and electric rates as the inputs. We also recognized the importance of underutilized factors such as differing part-load chiller efficiencies and chiller parasitic loads (see Table 1). This, hopefully, would provide a more cost-effective algorithm for real-time sequencing decisions.

It is also important to recognize that for such a scheme to work, the utility rate schedules need to be continually updated in the control system as they change.

The algorithm

The first step in developing the algorithm involved defining potential chiller-operation combinations. As previously noted, the plant had four chillers in parallel—two 1,000-ton electric centrifugal chillers and two 500-ton direct-fired double-effect gas absorption chillers (the potential chiller combinations are noted in Table 2 on p. 44).

Once the various combinations were defined, chiller loading for each option could be determined for a given plant load. Because the chillers in this plant were configured in parallel, all operating units will load to the same percentage of chiller load. For example, with a load of 1,200 tons, chiller combination #4 would load each chiller to 80%—800 tons on the electric chiller and 400 tons on the gas-driven chiller.

The electric centrifugal chiller efficiencies could be estimated from manufacturers' test data—yielding full- and part-load kW/ton values—utilizing available Air-Conditioning and Refrigeration Institute (ARI) Standard 550 Non-Standard Part-Load Values (NPLV). The direct-fired gas absorption chiller efficiencies can also be estimated from manufacturers' test data—which in this instance yields coefficient of performance (COP) values—utilizing available ARI 560 Integrated Part-Load Values (IPLV).

For example, the electric centrifugal chiller manufacturer's efficiency data indicates a concave-shaped trend. So according to the data, the NPLV kW/ton reaches 0.534 at 100% load, drops to 0.416 at 50% load and increases again to 0.565 at 20% load. Conversely, the two-stage absorption chillers with gas-driven engines can be characterized by a convex shaped trend, where the IPLV COP is 1.0 at 100% load, rises to 1.154 at 50% load and thereafter falls to a value of 1.136 at 25% load. Note that software can be used to determine the polynomial equation that provides the best curve fit for the said data.

Additionally, parasitic electrical loads can be accounted for as a fixed electrical load when the chiller is operating. For both the electric and gas chiller, parasitic loads include the associated primary chilled-water pump and condenser water pump. The gas chiller also includes an electrical load for the burner blower, solution pumps and purge pump. As a result, the parasitic loads are 79.3 kW for the 1,000-ton electric centrifugal and 62.5 kW for the 500-ton direct-fired absorption chiller.

Learning from the results

Using the information from this analysis, the control system can now choose the optimum chiller combinations for a given load by estimating the operating cost per ton-hour for each feasible combination.

This algorithm must be able to recognize when a chiller combination is not feasible—for example, if chiller loads fall below the practical limits or when loads exceed the capacity of the chiller combination.

Table 3 shows the results of the control algorithm using three different electrical rates with a constant $0.35/therm gas rate. The algorithm determines the chiller combination with the lowest predicted operating cost to automatically determine the best sequencing strategy based on utility cost data. Note that at $0.05/kWh, and with a load of less than 1,000 tons, it is more cost effective to use electric chillers only. The results also show a dramatic increase in operating costs when on-peak electric costs hit $0.18/kWh and the plant experiences loads beyond 1,000 tons. This, of course, is due to the 1,000-ton capacity limit of the installed gas absorption chillers.

Not shown on the table, but important to note, is that the electric parasitic loads on the gas absorbers accounted for 26% to 30% of the overall operating costs (cost/ton-hour), demonstrating that parasitic loads are an important consideration when determining the control logic.

Taking full advantage

Overall, current control-system technology offers the opportunity to implement more complex calculation logic for optimizing hybrid plant operation. This advance may allow plant operators to minimize operating costs many years after the design team has left the project.

To accomplish this setup, however, it is important to remember that the system designers must be willing to detail the necessary control logic in the sequence of operations. Otherwise, control-system programmers will not be able to effectively implement them.

Table 1 - Chiller Controls: Conventional vs. Alternative Approach

Conventional (prescribed) sequencing based on: Alternative (dynamic) sequencing accounting for:
time-of-day scheduletime-of-day schedule
chilled-water supply temperaturemeasured plant load (tons)
chilled-water return temperaturechiller efficiency data (kW/ton or COP)
chiller parasitic loads (kW)
electrical cost ($/kWh)
gas costs ($/therm)

Table 2 - Potential Chiller Combinations

Chiller Combination Chillers Operating Capacity (tons)
1(1) 500-ton gas500
2(1) 1,000-ton electric1,000
3(2) 500-ton gas1,000
4(1) 500-ton gas & (1) 1,000-ton electric1,500
5(2) 1,000-ton electric2,000
6(1) 1,000-ton electric & (2) 500-ton gas2,000
7(2) 1,000-ton electric & (1) 500-ton gas2,500
8(2) 1,000-ton electric & (2) 500-ton gas3,000

Table 3 - Chiller Algorithm Output

Example 1 Example 2 Example 3
Gas ($/therm)$ 0.35$ 0.35$ 0.35
Electric ($/kWh)$ 0.05$ 0.12$ 0.18
Plant Load (Tons) Best Chiller Combination Cost/Ton-Hr Best Chiller Combination Cost/Ton-Hr Best Chiller Combination Cost/Ton-HR

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