The restructuring of the utility industry adds complexity to the issue of HVAC design. The typical chiller system is one that is all electric, but trends in electric utility pricing indicate that estimating the cost of electricity will be increasingly difficult during the energy system design phase. Even today, larger industrial and commercial customers can have real time pricing electric rates that vary hour to hour, and these rates are not known until 4 PM the preceding day. The natural gas market is so volatile that consumers are now purchasing gas on the futures market to reduce risk. In this uncertain environment of energy pricing, one can only look at trends. Electric rates will continue to be relatively higher during the periods of time when demand is high (summer hot afternoon peaks). Conversely, trends in natural gas pricing indicate that natural gas prices will continue to be relatively low in the summer when demand is generally lower. Pipeline capacity to deliver the gas will be sold on a free market basis and will also be available at a lower cost during these same summer periods. These trends and uncertainties produce the need for operational flexibility by the energy purchaser. HVAC designers and building operators need to design and install equipment that is able to take advantage of the changing energy market over the typical twenty year life of the chiller system.
Currently, there is a growing and improving variety of gas cooling equipment that could be used alone or in combination with an electric chiller to meet building cooling needs. Using an electric chiller in combination with a natural gas chiller, a hybrid chiller system, could take advantage of pricing opportunities in the volatile energy market. However, equipment and maintenance costs must be considered. The first time costs of the gas cooling equipment are higher than an electric chiller. Expected savings in energy costs with these gas or hybrid gas/electric systems should be enough to overcome the first time and maintenance costs in order to be advantageous over the traditional electric chiller system. In many cases, gas utility companies have offered rebates on the purchase of gas cooling equipment. These cases can result in consumers with lower energy bills.
In multiple and in hybrid chiller systems, the question arises as to how to operate the chillers; e.g., which chiller to operate as the baseloaded (lead) chiller and which chiller to operate as the peaking (lag) chiller. This kind of operating strategy plays an extremely important role on the economics of the system. Energy costs will be affected as well as maintenance costs because the baseloaded chiller will require more maintenance than the peaking chiller.
In order to study and determine the best operating strategies and the energy costs of chiller systems, it is necessary to develop system and component simulation models. These components are the chiller, the cooling tower and cooling tower fan, and the condenser water pump and associated piping.
The deregulation of both the natural gas and the electric utility industry necessitates the rethinking of HVAC design. Conventional total electric chiller systems may not be the most cost efficient in a deregulated environment. Hybrid systems, which combine the use of an electric chiller and a gas chiller, are the primary focus of this study, but single chiller systems and multiple chiller systems using two electric chillers or two gas chillers are also considered.
Weber (1988) developed an electric chiller system model focusing on the condenser side components. Component models were developed, tested, and verified with manufacturer's data. The objective of this study was to determine total annual energy consumption for the electric chiller/cooling tower system. Weber varied condenser water flow rates and cooling tower air flow rates in order to minimize annual energy consumption. The results indicated that the conventional practice of designing for a condenser water flow rate of 3 gpm per ton was too high. Also, the cooling tower air flow rate for the minimum energy use was lower than that obtained from the use of tower manufacturer's sizing charts.
Joyce's study (1990) was a progression of Weber's work by considering energy rates and looking at minimizing energy costs, including electrical peak demand cost. He focused again on the design of the condenser side components of the chiller. He varied cooling tower size, cooling tower air flow rate, condenser water flow rate, and the diameter of the condenser water pipe. Joyce also included energy rate schedules and weather data for Atlanta, New York, and Los Angeles to calculate annual system energy costs. He performed an economic payback analysis of the different design options. His results suggest the use of an even lower condenser water flow rate than Weber (1988) to produce lower construction and operating costs with no loss in performance. Joyce also concluded that optimal tower sizes were smaller and corresponding air flow rates were less than those used in conventional design practices.
Kirsner (1996) disagrees with Weber and Joyce and concludes that the standard 3 gpm per ton condenser water flow rate usually yields the lowest energy consumption for all the system components. He suggests that the lower energy costs that result from using lower flow rates highly depend on the specific local utility rates and chiller load profile.
Much of Joyce's work provided a foundation for Liu (1997), but he also studied the use of a floating temperature set point for the tower water entering the condenser. Liu used weather data and electric rates for the city of Atlanta. He concluded that the cooling tower air flow rate was excessive and that the use of a fixed minimum set point for the tower water outlet temperature between 65oF and 68oF provided the lowest annual energy consumption and energy costs. Liu updated and improved the chiller model by using more recent manufacturer's data. He based his cooling tower model on the work of Braun (1988). Braun's effectiveness model for the cooling tower was chosen to improve the solution speed of the tower model.
Much work has been conducted in studying the operating strategies of chiller plants. In "Optimum Chiller Loading", Austin (1991) suggests that there are substantial electrical cost savings when operating the most efficient combination of lead and lag chillers. He suggests that a lag chiller should be started only when two chillers can operate more efficiently than one. In most situations, this point occurs only at full load, but should be based on efficiency ratings at constant condenser water temperature. Austin does not agree with the rule-of-thumb that the optimum chiller efficiency is 60 to 80% load and even finds that most cases show increasing efficiencies up to 100% load. Austin states that even if a chiller's optimum efficiency is at 80% load, it is much more efficient to operate it alone rather than to operate two chillers at 40% load. Austin uses typical manufacturer's chiller curves and specific plant operating procedures to reach his conclusions. He also explores the history of part load efficiencies. Austin also recommends that since it is difficult to determine a chiller's part load performance curve for specific conditions, field testing is essential in knowing the optimal loading point for a specific chiller.
Beyene (1995), in "Performance Evaluation of Conventional Chiller Systems", concludes that the number of chillers at a given site, the weather profile, and the load distribution have substantial influence on chiller energy consumption. He recommends the use of a penalty factor (PF) to quantify the influence of these variables on chiller performance. With the use of the PF, he concludes that a single centrifugal chiller operates the most inefficiently (it has the highest PF). Two and three centrifugal chillers have about the same PF. The difference in energy consumption among sites that utilize multiple chillers is not as significant as the energy consumption increase in single over multiple chiller sites. He shows agreement with field data and simulation results. Beyene also concludes that the advantages of multiple chillers diminish after three units.
There is a growing and improving variety of gas cooling equipment that has been studied recently and compared to their electrical counterparts. Tozer (1994) compared the operating costs of large absorption chillers to centrifugal chillers. He concludes that the absorption chillers are in good position to compete with centrifugal chillers because the absorption unit is competitive in running costs with respect to the centrifugal chiller for all different types of buildings. With high demand charges, electric costs can be expensive. Tozer also suggests that absorption chillers can offer improvement with respect to global warming potential, acid emissions, and ozone-depleting potential.
Katz (1995) suggests the use of natural gas cooling equipment to replace the electric chillers for environmental reasons. Natural gas engine-driven systems use hydrochlorofluorocarbon or hydrofluorocarbon refrigerants, which are more benign than chlorofluorocarbon refrigerants used in more than 80% of centrifugal chillers. Absorption chillers operate completely differently and use an environmentally friendly combination of water as the refrigerant and a solution like lithium bromide as the absorbent. Katz also suggests that the restructuring of the electric industry should open opportunities for gas cooling. Even though deregulation predicts that overall electric prices will go down, winter electric rates will tend to decrease while summer electric rates will tend to increase. There will be a summer price advantage for natural gas. Katz also indicates that, with the use of gas cooling equipment, the electric utilities could avoid the costs of building expensive new "peaking generating capacity" to meet high summer demand.
Examples of the benefits of gas engine driven chillers in specific applications are plentiful. One article in "Engineered Systems" (1995) investigates the installation of a 250 ton gas engine driven chiller in a JC Penny store in Atlanta. The store was projected to save over $54,000 each year in energy costs. Another article, by Procell (1996), studies the replacement of an aging centrifugal chiller with a gas engine driven chiller for a large apartment building. An energy study was completed to determine the best choice out of four options: 1) modifying the existing chiller with a new refrigerant, 2) installing a new high-efficiency electric-drive centrifugal chiller, 3) installing a natural gas engine driven chiller, or 4) installing a low-pressure steam driven absorption chiller. The findings of the energy study lead to the decision of purchasing the gas engine driven chiller. Even though the first time costs were less expensive for the electric chiller, the local utility company offered a rebate on the purchase of a gas engine driven chiller, which has proven to have lower energy costs than the older electric chiller. In another article, Randazzo (1996) investigates the installation of a hybrid system at a medical center in South Carolina. The chiller plant consists of a gas engine driven chiller and a centrifugal electric chiller. The gas engine driven chiller is assumed to carry the base load while the centrifugal unit is used as needed for peak cooling. The hybrid cooling plant was expected to see a payback over a more conventional cooling plant of three smaller electric chillers in eight months.
Wylie and Alvarez (1997) investigated the potential of a hybrid central plant. They suggest that even though new opportunities for gas cooling have been created with the advent of new and improved double stage absorption chillers, fuel price, efficiency, maintenance, and first-cost might hinder this opportunity. They only considered, however, absorption chillers and not gas engine driven chillers.