|
|
|
||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|||
|
|
|
|
|
In Figure 6.1, the equipment costs of all of the chiller systems considered in this study are graphically compared.
It can be seen that the 1000 ton electric chiller is the least expensive of the chiller systems considered for this study and, therefore, can be considered to be the base case for simple payback calculations. The most expensive chiller system is the one that consists of two 500 ton engine driven chillers.
| Pump and Fan |
Chiller (W/O Heat Rec) |
Chiller (W/ Heat Rec) |
Maintenance | Total Operating |
|
| 1000 Ton Electric Chiller | $ 15,979 | $ 37,818 | - | $ 15,000 | $ 68,797 |
| 1000 Ton Single Stage Absorption Chiller | $ 21,249 | $ 126,162 | - | $ 12,000 | $ 159,411 |
| 1000 Ton Double Stage Absorption Chiller | $ 32,220 | $ 56,618 | - | $ 15,000 | $ 103,838 |
| 1000 Ton Gas Engine Driven Chiller | $ 14,917 | $ 35,539 | $ 14,808 | $ 23,220 | $ 52,945 |
| 500 Ton Electric Chiller (Lead) with 500 Ton Single Stage Absorber |
$ 6,773 $ 4,768 |
$ 32,136 $ 13,879 |
- - |
$ 13,500 | $ 71,056 |
| 500 Ton Electric Chiller (Lead) with 500 Ton Double Stage Absorber |
$ 6,773 $ 4,765 |
$ 32,136 $ 6,367 |
- - |
$ 15,000 | $ 65,041 |
| 500 Ton Electric Chiller (Lead) with 500 Ton Gas Engine Driven Chiller |
$ 6,773 $ 3,131 |
$ 32,136 $ 3,429 |
- $ 1,429 |
$ 9,896 | $ 53,365 |
| 500 Ton Electric Chiller (Lead) with 500 Ton Electric Chiller |
$ 6,773 $ 2,981 |
$ 32,136 $ 6,006 |
- - |
$ 15,000 | $ 62,896 |
| 500 Ton Engine Driven Chiller (Lead) | $ 6,350 | $ 30,976 | $ 12,908 | ||
| 500 Ton Engine Driven Chiller (Lead) with 500 Ton Gas Engine Driven Chiller |
$ 6,350 $ 3,131 |
$ 30,976 $ 3,429 |
$ 12,908 $ 1,429 |
$ 23,220 | $ 47,038 |
| 500 Ton Engine Driven Chiller (Lead) with 500 Ton Electric Chiller |
$ 6,350 $ 2,981 |
$ 30,976 $ 6,006 |
$ 12,908 - |
$ 28,325 | $ 56,570 |
Figure 6.2 compares the annual energy costs of the single chiller systems. The single stage absorption chiller's energy costs are far too expensive to be considered a competitive alternative to the 1000 ton electric chiller. Its energy cost is nearly three times more than the 1000 ton electric chiller or the 1000 ton engine driven chiller.
The 1000 ton double stage absorption chiller is more expensive than these two systems also. Its energy cost for the pump and fan is double that of the same components for the engine driven and electric chillers. The double stage absorber requires much more pumping power than any of the other chillers studied. This is due to the unusually high pressure drop across the condenser and absorber, which may be particular to the specific manufacturer. A smaller head loss through the condenser/absorber of the two-stage absorber would require less pumping power; and, therefore, the cost for pumping would be less. This could make the double stage absorber a more attractive option.
The gas chiller that is most competitive with the electric chiller is the 1000 ton engine driven chiller. This chiller has lower energy costs, and, with the heat recovery option, the energy costs drop dramatically. The annual energy cost for the 1000 ton gas engine driven chiller drops from $50,456 (6.6% lower than the 1000 ton electric chiller) without heat recovery to $29,725 (81% lower than the 1000 ton electric chiller) with heat recovery. However, the annual operating cost for the 1000 ton gas engine driven chiller with heat recovery is $52,945 (only 30% lower than the annual operating cost for the 1000 ton electric chiller due to the high maintenance cost).
The equipment cost of the 1000 ton gas engine driven chiller is $540,000, much higher than that of the 1000 ton electric chiller, which is $175,900. If the site is not able to take advantage of the heat recovery option offered by the gas engine driven chiller, there is not much advantage to the gas engine driven chiller. Even though the annual energy savings of the 1000 ton gas engine driven chiller with heat recovery is $27,604, the payback is still about 23 years due to high maintenance and first time costs of the total engine driven chiller system.
All of the multiple chiller systems, except the 500 ton electric chiller in combination with the 500 ton single stage absorption chiller, have less expensive energy costs than any of the single chiller systems. This is primarily due to the savings in pumping power. Instead of one larger pump, there are two smaller ones operating at a lower flow rate. Even though these multiple chiller systems have higher equipment costs than single chiller systems, they are generally preferred so that the chiller site will have redundancy and backup. A multiple chiller system is also preferred for maintenance reasons. During the low demand times, the building operator can take one chiller down for maintenance and still have the other one operating.
With heat recovery from the engine driven chillers, any of the multiple chiller systems incorporating a gas engine driven chiller have dramatically reduced energy costs. The annual energy costs of the hybrid system consisting of a lead electric chiller with a lag engine driven chiller reduce from $45,469 without heat recovery to $43,469 with heat recovery. The annual energy costs of the system of two gas engine driven chillers reduce from $43,886 without heat recovery to $23,818 with heat recovery. The energy costs of the hybrid system of a lead gas engine driven chiller with a lag electric chiller decrease from $46,313 without heat recovery to $28,245 with heat recovery.
Since the absorption chillers have little or no energy savings over the 1000 ton electric chiller and have higher equipment costs, this study will consider only the last four chiller systems as competitive alternatives to the 1000 ton electric chiller.
If the simple payback calculations are performed against the multiple chiller system consisting of two 500 ton electric chillers, then the payback period for this hybrid chiller system is 18.4 years.
If the simple payback calculations are performed against the multiple chiller system consisting of two 500 ton electric chillers, then the payback period for this chiller system of two 500 ton gas engine driven chillers is 22 years.
If the simple payback calculations are performed against the multiple chiller system consisting of two 500 ton electric chillers, then the payback period for this hybrid chiller system of a lead gas engine driven chiller with a lag electric chiller is 28 years.
The most competitive alternative to the 1000 ton electric chiller, which has the lowest equipment cost, for meeting the cooling requirements of the building considered in this study is the system consisting of two 500 ton electric chillers. This system has an annual energy and operating savings over the 1000 ton electric chiller of $5,901. Its simple payback is 9 years. The most competitive hybrid chiller system is the one that incorporates a 500 ton lead electric chiller and a 500 ton lag gas engine driven chiller. This system has an annual energy saving over the 1000 ton electric chiller of $25,552, but its operating cost is only $15,432 lower than the operating costs of the 1000 ton electric chiller. Since its equipment and maintenance costs are so much higher, the simple payback is 14.8 years vs. the 100 ton electric chiller and 18.4 years vs. the two 500 ton chillers.
The electric and gas rates used in this study are representative of deregulated energy rates. Even though these rates may go as high as $0.80/kWh during the summer, this rate varies hourly and is only that high for a few hours at most. With the older, regulated electric rates, the gas engine driven chillers would have shown even greater energy savings over the electric chiller, but with these new rates, the electric chiller's economics have improved. With deregulated rates, the economic advantage of the gas chillers decreases. The payback period for the two 500 ton electric chillers is 9 years but could be preferred over the 1000 ton electric chiller for maintenance and backup reasons.
This study only considers hybrid chiller systems that consist of equally sized chillers. Other non-equal sized hybrid chiller systems should be studied; e.g., a hybrid system consisting of a 700 ton electric chiller and a 300 ton gas chiller or vice versa. These types of systems may yield different results.
Other operating strategies should also be considered; e.g, alternating which chiller operates as the lead chiller and which chiller operates as the lag chiller in a multiple chiller system. Better results might be obtained by baseloading the electric chiller in the winter when electric rates are relatively lower and baseloading the gas chiller in the summer when gas rates are relatively lower. Alternating the lead and lag chiller could also save on the maintenance costs by evening out the wear on the chillers.
It should also be noted that the results presented in the current study are highly dependent on weather data and energy rates. For this study, the only location and energy rate schedules that were considered were for the city of Atlanta. The results would be influenced by using the weather data and energy rates for another geographical location.
Finally, as deregulation of the utility industry progresses, the developing deregulated energy rates should be studied for more accurate and up-to-date energy costs for the various chiller systems.