Control Optimization for a Chilled Water Thermal Storage System Under a Complicated Time-of-Use Electricity Rate Schedule

Transcription

1 4759 ESL-PA Control Optimization for a Chilled Water Thermal Storage System Under a Complicated Time-of-Use Electricity Rate Schedule Jijun Zhou Member ASHRAE Song Deng, PE Member ASHRAE Guanghua Wei, PE Associate Member ASHRAE David E. Claridge, PhD, PE Member ASHRAE W. Dan Turner, PhD, PE Oscar Contreras ABSTRACT The existence of a 1.4-million-gallon chilled water thermal storage tank greatly increases the operational flexibility of a campus-wide chilled water system under a fourprice time-of-use electricity rate structure. While significant operational savings can be expected, the complication in the rate structure also requires more sophisticated control over the thermal storage tank charging and discharging processes. A chiller start-stop optimization program was developed and implemented into the Energy Management and Control System (EMCS) to determine the number of chillers that need to be brought on line and the start and stop times for each chiller every day, based on the prediction of the campus cooling load within the next 24 hours. With timely and accurate weather forecasting, the actual tank charging and discharging process closely matches the simulated process. The chiller plant s operating schedules are dynamically optimized to deliver required cooling capacity at lowest possible operating costs. INTRODUCTION Thermal storage is economically attractive under a timeof-use electricity rate schedule. A typical use of the chilled water thermal storage tank is to charge the tank during offpeak hours when utility costs are relatively low and discharge the tank during on-peak hours to reduce operating costs. Equally significant savings may be achieved if a facility is subject to demand charges, in which case the thermal storage tank is used to level-off the peak demand in order to avoid hefty demand charges. Another less obvious benefit of the thermal storage is the decoupling of the thermal load profile from the operation of the equipment, adding an element of flexibility and reliability to the system [ASHRAE 23]. While it is conceptually simple to take advantage of the thermal storage tank, the actual control strategy for a chilled water system with a thermal storage tank may turn out to be rather complicated, especially if multiple chillers are involved and the facility is under a complicated utility rate schedule. Even when the capacity of the thermal storage tank is big enough to serve the cooling load for the entire on-peak period, needs for operational optimization still exist and the tank should be used only to the extent necessary since unnecessary production of the storage almost always introduces extra cooling load [Tamblyn 1985]. Many papers have been published in the last two decades on the subject of thermal storage, including several which emphasize the operation and control issues [Shavit 1985 and Fiorino 1991]. Few sources, however, discuss in detail the optimization process for facilities under a complicated time-of-use utility rate structure. Unsuccessful thermal storage projects are seldom documented but do exist. Liu [1999] described how rehabilitation was able to improve the performance of an unsuccessful storage system and reduce the operating costs. Many other case studies can be found in literature, some of which details a step-by-step control optimization procedures for a specific electricity rate schedule [Wei 22]. A complete control strategy for a thermal storage system describes the sequences of operation under all possible operating modes, such as charging storage, charging storage while meeting load, meeting load from discharging, meeting load from discharging and direct equipment operation, demand-limiting control, etc [ASHRAE 23]. The most economic operating schedule has to be determined with many factors in mind, such as load prediction, utility rates, demand charges, capacities of the chillers and the thermal storage tank. J. Zhou is a research associate, G. Wei and S. Deng are assistant directors of the Energy Systems Laboratory, and W.D. Turner and D.E. Claridge are professors in the Department of Mechanical Engineering, all at Texas A&M University, College Station, Tex. O. Contreres is HVAC supervisor of Texas A&M University at Corpus Christi, Tex. 25 ASHRAE.

2 Chiller ESL-PA Chillers Constant Speed Pumps P T Chiller Chiller Storage Tank P T Expansion Tank Variable Speed Pumps Figure 1 Central plant chilled water loop and the thermal storage tank. The case study facility is a university campus located in south Texas. The weather is very hot and humid in the summer and warm and humid in the winter. The campus has over one million square feet of conditioned space, including class- rooms, laboratories, offices, student service centers, library, gyms and research facilities. The entire campus peak cooling load is estimated at 18 tons (6331 kw) in 21. Three 1-ton (3517 kw) centrifugal chillers located at the central plant supply chilled water to the campus. Under design conditions, each chiller supplies approximately 17 gpm (17 L/s) of chilled water at 42 F (5.6 C), with a return water temperature of 56 F (13.3 C). The chilled water system is a typical primary-secondary loop configuration with constantspeed primary pumps and variable-speed secondary pumps. The thermal storage tank is situated on the floating bypass, as shown in Figure 1. Without any additional control, the thermal storage tank acts much like a natural reservoir it charges when the total chiller production exceeds the chilled water demand of the campus, and discharges when the production is less than the demand. The thermal storage tank is 64 ft (19.5 m) tall and 61 ft (18.6 m) in diameter with a total volume capacity of 1,4, gallon (5,299,56 L). Under design conditions, the fully charged thermal storage tank can hold a cooling capacity of 1 ton-hr (42,24 kwh). The temperatures of the stratified chilled water layers are monitored by 3 temperature sensors evenly distributed along the vertical dimension of the tank. At the beginning of the year 22, a four-tiered utility rate schedule was accepted by the case study facility. The new rate structure was considered to be more cost effective comparing to other potential rate structures, including a flat rate and a two-tiered rate schedules. However, maximum cost savings can be achieved only if the chillers and the thermal storage system can be operated in the most efficient way to take full advantage of the lower-price periods. The new rate schedule is different from season to season and is divided into four parts, with each part covering several months. There are as many as four different electricity prices at different times of a day. In April, October and November, the facility is charged for price-1 (the lowest electricity price) only at any time of the day. From December to March, two prices (price-1 and price- 2) are included in the rate schedule. May and September will see three electricity prices at different times of the day. The peak summer period from June to August has the most complicated rate schedule of all, which includes all the four electricity prices. Except for April, October and November, weekends have different rate schedules from weekdays. Detailed price tiers for different hours in each month are tabulated in Table 1. From October to April, the economic operation of the chiller plant is relatively easy compared to the remaining warmer months. In particular, since only one price (price-1) is involved in the rate structure for weekdays of October, November, and April and the weekends of all the months from October to April, there is essentially no need to control tank charging and discharging. However, due to the inevitable heat loss from the thermal storage tank, the cooling capacity charged into the tank should be kept at the minimum level. For the weekdays of the months from December to March, the price-2 periods are separated by 6 hours of price-1 period, and there is no difficulty in avoiding running chillers during the price-2 periods.

3 Table 1 Four-Tier Electricity Rate Schedule DEC - MAR APR, OCT & NOV MAY & SEP JUN - AUG Time M-F SS M-F SS M-F SS M-F SS : : : : : : : : : : : : : : : : : : : : : : : : Note: Price 1-$.298/kWh, Price /kWh, Price 3 - $.5187/kWh, Price 4 - $.7333/kWh. The situation is more complicated for the remaining warm months. As mentioned earlier, the fully charged thermal storage tank can hold approximately 1 ton-hr (42,24 kwh) of cooling capacity and the campus peak cooling load observed was around 1,8 tons (6331 kw). Therefore, it is obvious that the campus can solely depend on the thermal storage tank (without running any chiller) during the six-hour price-4 period (from 2 p.m. to 8 p.m.), provided that the tank is fully charged at 2 p.m. After that, the tank may still have enough cooling capacity left to provide cooling to the campus for up to several hours, depending on the actual cooling load of the day. The chillers should be brought on timely before the tank is depleted to meet the campus cooling demand; meanwhile, they should also be stopped in time to avoid running into higher-price time periods. The tank should be charged to a specified level so that it can provide chilled water for as many higher-price hours as possible; however, it is not always desirable to have a full tank if the campus load is relatively low. The number of chillers that should be brought online during the charging cycle and the exact times to start/stop a specific chiller should be determined accurately to take full advantage of the fourtiered rate structure. PROCEDURES TO OPTIMIZE CHILLER STARTS AND STOPS The exact chiller start/stop schedules will vary from day to day. It is obvious that, for the months from May to September, the thermal storage tank should be charged during night time when the electricity price is lower and be discharged in the afternoon when the electricity price is higher. Assume the tank is almost depleted at 1 p.m., and one or more chillers would be brought online in sequence. If the chiller(s) produce more cooling than what is demanded by the campus at the time, the tank would be gradually charged. The tank would then be fully charged sometime in the early afternoon, before the higherpriced period starts. Then all the chillers would be shut down

4 and the tank starts to discharge. At around 1 p.m., the thermal storage tank would be almost depleted again and a new tank charging cycle starts. This tank charging/discharging sequence is conceptually straightforward. However, the exact times for starting and stopping the chillers that will take full advantage of the thermal storage tank and the four-tiered rate schedule can only be determined from more detailed analysis. The optimal chiller start/stop sequence varies from day to day depending on the campus actual cooling load. A chiller start/stop optimization (CSSO) program was developed and implemented in the Energy Management and Control System (EMCS) to automate the decision-making process to determine the most economic operating schedule of the central chiller plant each day, based on many factors and restraints. Unless otherwise stated, the CSSO procedures discussed in this paper address the warmer months when at least three different electricity prices are involved in a single day. The procedures for the chiller start/stop optimization include the following steps. First, the cooling load of the campus is estimated for the next 24 hours with a load prediction model based on the weather forecast. Next, the best period of time for the tank to discharge is selected, including as many higher-priced hours as possible. The time period to charge the tank is automatically determined as well. Then, the average chiller production rate required during the charging period is calculated from the total estimated campus load and the available charging hours. After that, the number of chillers to be brought online and the runtimes for each chiller are determined based on the predicted campus load. Finally, the start/stop times for each chiller are scheduled precisely, after taking many restrictions and specific requirements into consideration. Campus Load Prediction Model The total cooling load of all the buildings on the campus chilled water loop is regarded as a simple function of the outside air temperature. At any time, this total cooling load can be estimated from the instantaneous chilled water consumption rate at the central chiller plant, which can be calculated from the flow rate, the supply and the return temperatures of the chilled water that are continuously monitored and trended by the EMCS. Unfortunately, the chilled water flow meter was not working properly at the time and therefore there was no direct means to measure the campus chilled water consumption. The chilled water supplied to the campus comes from two sources, the chillers and/or the thermal storage tank. When the chillers produce more than what the campus needs, the excess chilled water flows through the thermal storage tank and charges it; when the chillers produce less than needed, the tank discharges to help meet the demand; when all chillers are shut down, the tank provides all the needed chilled water to the campus. Under the last scenario, the campus cooling load can be evaluated from the tank s discharging rate. Fortunately the thermal storage tank itself is metered adequately, with a total of 3 temperature sensors evenly distributed along its vertical dimension. From the overall temperature changes within the tank during certain periods of time, the rate at which the tank was discharged (or charged) can be calculated. Figure 2 and Figure 3 show the thermal storage tank s discharging (or charging) rate calculated from the trended data, for weekdays and weekends respectively. A positive value indicates the tank was discharging and a negative value indicates the tank was being charged. On both figures, clear separations among data are shown. The top cloud of data on each figure shows the operation scenario when all the chillers were shut down and the campus chilled water was solely supplied by the tank itself. Therefore, the discharging rates are all positive. These data points actually outline the campus 2,5 2,5 Tank Discharging Rate (Tons) 5-5 Tank Discharging Rate (Tons) Ambient Temperature (deg F) Ambient Temperature (deg F) Figure 2 Thermal storage tank discharge rate (weekdays). Figure 3 Thermal storage tank discharge rate (weekdays).

5 Campus Cooling Load (Tons) Weekday Weekend Ambient Temperature (deg F) Figure 4 Campus cooling load prediction model. Ambient Temperature (deg F) /11/2 6: 6/11/2 12: 6/11/2 18: 6/12/2 : Ambient Temperature Model Actual Hourly Temperature 6/12/2 6: 6/12/2 12: Time 6/12/2 18: 6/13/2 : 6/13/2 6: 6/13/2 12: 6/13/2 18: 6/14/2 : Figure 5 Modeled ambient temperature profile vs. actual hourly weather data. cooling load profiles for weekdays and weekends. The middle cloud of data shows the operation scenario when one chiller was running and the tank may discharge or be charged depending on the campus load. Similarly, the bottom cloud of data was for the operation scenario when two chillers were running, and the tank would always be charged since the campus load never exceeded the capacity of two fully loaded chillers. For simplicity, the campus cooling load profiles for weekdays and weekends are modeled with simple three-point-change-point (3PCP) linear models, as shown in Figure 4. The ambient temperature profile for the next 24 hours is generated from the weather channel s daily weather forecast. The temperature profile is generated with a sine function, which peaks at 3 p.m. and bottoms at 3 a.m. The predicted ambient temperature at any hour is calculated as T n T = hi + T 2 lo T + hi T 2 lo n 9 Sin( π ) 12 Where T hi = daily high temperature from weather forecast T lo = overnight low temperatures from weather forecast n = the hour of the day (from to 23) Figure 5 shows the comparison between the generated ambient temperature profile and the actual hour-by-hour weather data from the weather channel. The close match between the two indicates the ambient temperature prediction model is sufficiently accurate. Determination of Daily Tank Charge/Discharge Periods With the campus cooling load predicted for the next 24 hours, the total required chilled water production could be determined. The next question to address is when and how long to run the chillers and for how long, or in other words, to determine the tank charging/discharging periods. One also has to remember that the tank may be discharging even when one chiller is running. However, the tank discharging period is defined as the period when all the chillers are offline and the campus cooling is solely supplied by the tank itself. The rest of the time when at least one chiller is running is defined as the tank-charging period. Since it is advantageous to use (discharge) the thermal storage tank during higher-price periods, the tank discharge period is selected hour-by-hour starting from the most expensive hours to the least expensive hours. For the months from June through August, when all the four electricity prices come into play, the hours to use the tank (and therefore avoid running chillers) should be selected in the sequence shown below. Accumulate the predicted campus cooling loads hour-byhour, in the above sequence, until the maximum tank capacity is reached. For the rare cases, the accumulated cooling load for the most expensive 12 hours (i.e., from 1 a.m. to 1 p.m.) may be still less than the tank capacity. What to do next depends on the month. For May and September, since all the rest of hours are charged for the cheapest rate (price-1), the tank only needs to store enough chilled water for those 12 hours. The tank should start discharging at 1 a.m. and start the charging process at 1 p.m. For June, July and August, however, the tank should be Price 4 Price 3 Price 2 Price 1 7 a.m. 6 a.m. 5 a.m. 4 a.m. 3 a.m. 2 a.m. 1 a.m. 12 a.m. 11 p.m. 1 p.m. 8 a.m. 9 a.m. 1 a.m. 11 a.m. 12 p.m. 1 p.m. 9 p.m. 8 p.m. 7 p.m. 6 p.m. 5 p.m. 4 p.m. 3 p.m. 2 p.m.

6 exploited further. There is another four hours (8 a.m. to 1 a.m. and 1 p.m. to 12 a.m.) of price-2 period that can take advantage of the thermal storage tank. Keep accumulating the hourly cooling loads for the price-2 hours, until the tank capacity is reached. The discharging process starts at the earliest selected hour and ends at the latest selected hour. The weekends are different from the weekdays due to the difference in rate schedules. For May and September, only the eight hours from 2 p.m. to 1 p.m. are charged at price-2 and all other hours are charged at price-1. The tank should only be charged to the amount required by these eight hours. The weekends of June, July and August are charged at three prices daily. Therefore, similar methods as described for the weekdays should be used to determine the time period for the tank discharging process. The tank charging process starts as soon as the tank discharging process is over; when the tank charge is expected to be depleted (or almost depleted). Because the physical plant requires the chillers to be started under operators observation, the actual tank charging process should be started before the operator on the last shift leaves duty, which is around 1:3 p.m. For that reason, the latest time to stop the tank discharging cycle (and to start the tank charging cycle) was set at 1 p.m. The tank charging cycle ends at the start of the tank discharging cycle. Total Predicted Chiller Load and Chiller Run Time The total predicted chiller load (L p ) is defined as the load seen by all the chillers for the next charging cycle, which is essentially the total estimated campus load (L c ) for the next 24 hours, unless the tank is not depleted at the start of the charging cycle. In that case, the total predicted chiller load is the estimated total campus load less the remaining cooling capacity in the tank (L t ), plus the backup load (L b ). L p = L c - L t + L b Having known the total estimated chiller load and the length of the tank-charging period (H chg ), the average chiller load (R p ) during the tank charging cycle can be determined as R p = L p / H chg And the total chiller runtime (H tot ) can be calculated as H tot = L p / C chlr Where C chlr = single chiller full load capacity Depending on the calculated average chiller load, one, two or all three chillers may need to be turned on during the charging cycle. Chiller Start/Stop Optimization Scenario 1: All (three) chillers. If R p is greater than two times a single chiller s full load capacity (C chlr ), two chillers (the lead and the lag chillers) will need to run for the entire charging period, and the third chiller (the backup chiller) will also need to run for a certain period of time. The runtime for the backup chiller H bak is calculated as H bak = H tot - 2 * H chg The chiller start/stop sequence is determined as: Both the lead and the lag chillers will be turned on at the start of the tank charging cycle (τ cs ), and shut down at the end of the tank charging cycle, or the start of the tank discharging cycle (τ ds ); The backup chiller starts at τ cs, and stops at (τ cs + H bak ). Scenario 2: One chiller only. If R p is smaller than one chiller s full load capacity, only one chiller needs to run for a fraction of the determined charging period. In that case, the actual tank charging period will be less than what is already determined, since the chillers are always fully loaded during the charging cycle. The actual charging hours is the calculated total chiller runtime: H chg = H tot The chiller start/stop sequence is determined as: The lead chiller starts at τ c, and stops at the (τ cs + H chg ); Neither the lag chiller nor the backup chiller will be started. Scenario 3: Two chillers. If R p is greater than the capacity of a single chiller and less than that of two chillers, the lead chiller will be turned on for the entire charging period and the lag chiller will run for a fraction of the charging period. The runtime for the lag chiller is calculated as H lag = H tot - H chg Obviously, the lead chiller starts at τ cs and stops at τ ds. The lag chiller will attempt to start at τ cs, and run for H lag hours. However, this may bring forth certain unexpected results in the tank charging process. Theoretically, the best charging process would be to partially load the lead and the lag chillers so that the total chiller production rate matches the calculated average chiller load R p. That way, both chillers run for the entire period of the charging process and the tank is charged smoothly from depleted to full. A simulated tank level charging process is shown as Theoretical Process in Figure 6. In reality, the chillers are simply fully loaded during the charging period once they are running. Therefore, the alternative to the ideal process is to run the lead chiller for the entire charging period and run the lag chiller for part of the charging period only, both fully loaded.

7 Two Chiller Scenario - Theoretical Process Tw o Chiller Scenario - Early Charge Process Campus Load Chiller Production Tank Level Campus Load Chiller Production Tank Level 2,5 1% 2,5 1% 9% 9% 8% 8% 7% 7% Tons 6% 5% 4% 3% Tank Level Tons 6% 5% 4% 3% Tank Level 5 2% 5 2% 1% 1% % % 18: 21: : 3: 6: 9: 12: 15: 18: 21: : Time 18: 21: : 3: 6: 9: 12: 15: 18: 21: : Time Two Chiller Scenario - Late Charge Process Two Chiller Scenario - Prefered Process Campus Load Chiller Production Tank Level Campus Load Chiller Production Tank Level 2,5 1% 2,5 1% 9% 9% 8% 8% 7% 7% Tons 6% 5% 4% 3% Tank Level Tons 6% 5% 4% 3% Tank Level 5 2% 5 2% 1% 1% % % 18: 21: : 3: 6: 9: 12: 15: 18: 21: : Time 18: 21: : 3: 6: 9: 12: 15: 18: 21: : Time Figure 6 Comparison of different charging / discharging scenarios. Assume the tank charging cycle is determined to start at 1 p.m. and stop at 2 p.m. of the next afternoon, which is a 16-hour charging period. Also assume the total chiller hours (H tot ) needed is 29 hours. Then, the lead chiller will run through the entire charging period (16 hours), while the lag chiller will only need to run 13 hours, fully loaded. Now the question is, which 13 hours should the lag chiller be engaged? Or does it really matter? The answer is yes the choice of when to start/stop the lag chiller is important. One of the most obvious sequences is to start the lag chiller at the same time as the lead chiller. In this case, the lag chiller is started at 1 p.m. and stopped at 11 a.m. the next morning (13 hours). This process is simulated and shown as the Early Charge process in Figure 6. The simulation shows the tank is charged to 1% level at around 9 a.m. and kept fully charged until 11 a.m., at which time the lag chiller cuts out. During this time period, the chillers are forced to load partially since the tank is full and the campus doesn t need two fully loaded chillers. Starting from 11 a.m., the tank level starts to drop since the remaining lead chiller itself can no longer satisfy the campus cooling load. Under this scenario, the tank won t be fully charged at the end of the charging process (2 p.m.) and will be depleted earlier than expected (around 8:3 p.m.). Consequently, this forces the next charging process to be started as early as 8 p.m., instead of 1 p.m. This is an undesired scenario. Alternatively, the lag chiller could also be brought on at a later time (in this case, 3 hours later than the lead chiller, or at 1 a.m.) and kept running until 2 p.m. This is shown as the Late Charge process in Figure 6. Under this scenario, the campus load cannot be met from 1 p.m. till midnight since the campus load is still higher than what one chiller can provide. Besides, starting the lag chiller at 1 a.m. is unacceptable to this particular facility for the reason explained earlier. Therefore, this scenario is unacceptable. The only choice left is to break the lag chiller s runtime into two parts. One of the many possible schedules is to start the lag chiller at the same time as the lead chiller and shut it down sometime overnight at τ x (to be determined later). The lag chiller is then started again at 8 a.m., and kept running until the end of the charging period. In summary, the chiller start-stop sequence under this scenario is determined as: The lead chiller will be started at τ cs, and shut down at τ ds ; If the lag chiller s runtime (H lag ) is less than one hour shorter than that of the lead chiller (H led ), the difference

8 can be disregarded, and the lag chiller will be started at the same time as the lead chiller; Otherwise, if H lag is more than one hour less than H led, the two chillers will have different runtimes. The ideal time to start the lag chiller would be τ id = τ ds - H lag If τ id is later than 8 a.m., this will be the time to start the lag chiller; otherwise, the lag chiller runtime has to be broken into two parts. The second part is from 8 a.m. to τ ds. The first part is from τ cs to τ x, where τ x = τ cs + (H lag (τ ds 8)) The backup chiller will not be started. The complete flow chart of the chillers start-stop optimization program is shown in Appendix 1. SAMPLE CASE The scheduling process may be better illustrated by a real case. At 1 p.m. on July 4, 22, the thermal storage tank had around 95 ton-hours equivalent of chilled water left. From the weather channel, the morning low and daytime high for July 5th (Friday) were 8 F (26.7 C) and 9 F (32.2 C) respectively. For some unknown reason, the chillers were not able to deliver their designed capacity. Instead of supplying the designed 42 F (5.6 C) chilled water, these chillers were only able to supply 43 F (6.1 C) chilled water, with the design return temperature of 56 F (13.3 C) and chilled water flow of 172 gpm (18.5 L/s). Consequently, the chiller s full load capacity was around 932 ton (3278 kw), and the thermal storage tank s capacity was also reduced to around 11,84 ton-hours (41,641 kwh). Based on these conditions, the chiller start-stop sequence was to be determined for the next 24 hours. From the outside air temperature prediction model and the load model developed, the hour-by-hour ambient temperatures and corresponding campus cooling loads could be predicted for the next 24 hours, as shown in Table 2. Load factors had been developed as shown in the table to account for the reduced load during the summer school session and the reduced load from night setback (12 a.m. to 6 a.m.). From Table 2, the total cooling load for the next 24 hours was estimated as 28,119 ton-hr (98,895 kwh). The actual total chiller load of the next charging cycle is determined as L = L L + L p c e b where L c =total predicted campus cooling load for the next 24 hours, L e = the tank capacity remaining at the end of the last discharging cycle L b = the backup load, or 5 ton-hr (1759 kwh) in this case. The total chiller load was therefore calculated to be 27,669 ton-hr (97,312 kwh). To determine the tank discharging period, hourly loads were summed starting from the most expensive hours (tier 4) to the least expensive hours (tier 1) in this sequence: 2 p.m., 3 p.m., 4 p.m., 5 p.m., 6 p.m., 7 p.m., 8 p.m., 9 p.m., 1 p.m., 12 p.m., 11 a.m., 1 a.m. until the accumulated load reaches the tank capacity. From Table 2, it was determined that the tank (if fully charged) could provide the chilled water needs of the campus from 1:25 p.m. to 1 p.m. on July 5. It also means the tank charging period should be from 1 p.m. on July 4 (τ cs ), and stop at 1:25 p.m. on July 5 (τ ds ). The tank-charging period would last for 15.5 hours (H chg ). The calculated average chiller production rate (R p ) was 1785 ton (6278 kwh) and the total required chiller runtime (H tot ) was hours. Since R p was between the capacity of one chiller and two chillers, two chillers should be involved in the charging process. The lead chiller should run through the entire charging process, i.e., it should start at 1 p.m. on July 4 and stop at 1:25 p.m. on July 5. The runtime for the lag chiller was calculated to be hours. The lag chiller s runtime should be broken into two separated parts as described in the previous section, which may be called the overnight run period and the morning run period. The morning run period should start from 8: a.m. to the end of the charging process (1:25 p.m.), a period of approximately 5.4 hours. This leaves the overnight period for approximately 8.78 hours. The lag chiller s overnight run period should start from 1 p.m. on July 4, and stop at around 6:46 a.m. on July 5. According to chiller plant common operating practice, a certain period of time should be allocated between successive chiller start-stops. In this case, a 15 minutes time interval has been set. Taking this into account, the final chillers start-stop sequence was determined as follows: The lead chiller should start at 1:6 p.m. on July 4 and stop at 1:25 p.m. on July 5. The lag chiller should start at 1:22 p.m. on July 4 and stop at 6:5 a.m. on July 5. The lag chiller should start again at 8: a.m. on July 5 and stop at 1:41 p.m. on July 5. In reality, the backup chiller should be started instead of the lag chiller to avoid frequent chiller start/stops on the same chiller. With all the chillers start and stop times determined, the total chiller production at each hour could be calculated. Since the campus load had already been predicted for each hour as well, the tank charging and discharging process could be predicted for the next 24 hours. The hour-by-hour campus cooling load, chiller production and the chilled water level within the storage tank were predicted and shown in Table 3. The chiller start/stop program implemented in the control system automates the chiller scheduling sequence as described above and executes the chiller starts / stops. With the 3 temperature sensors installed on the thermal storage tank, the changes in the tank s chilled water temperatures at all levels can be monitored. At the end of each day, the accuracy of the tank charging/discharging process predicted 24 hours ahead can be

9 Table 2. Hour-by-hour Prediction of OAT and Campus Load for July 5, 22 Data/Time Rate Tier Predicted OAT Predicted Load Load Final Load Prediction F C Tons kwh factor Tons kwh 7/4/2 22: /4/2 23: /5/2 : /5/2 1: /5/2 2: /5/2 3: /5/2 4: /5/2 5: /5/2 6: /5/2 7: /5/2 8: /5/2 9: /5/2 1: /5/2 11: /5/2 12: /5/2 13: /5/2 14: /5/2 15: /5/2 16: /5/2 17: /5/2 18: /5/2 19: /5/2 2: /5/2 21:

10 Table 3. Predicted Tank Charge/Discharge Process for July 5, 22 Date/Time Predicted cooling load Produced chiller load Accumulated Tank Load Tank Level Ton kw ton kw ton-hr kwh ft m % 7/4/2 22: % 7/4/2 23: % 7/5/2 : % 7/5/2 1: % 7/5/2 2: % 7/5/2 3: % 7/5/2 4: % 7/5/2 5: % 7/5/2 6: % 7/5/2 7: % 7/5/2 8: % 7/5/2 9: % 7/5/2 1: % 7/5/2 11: % 7/5/2 12: % 7/5/2 13: % 7/5/2 14: % 7/5/2 15: % 7/5/2 16: % 7/5/2 17: % 7/5/2 18: % 7/5/2 19: % 7/5/2 2: % 7/5/2 21: % %

11 Tank Level 1% 9% 8% 7% 6% 5% 4% 3% 2% 1% % Measured Simulated 7/4/2 7/4/2 7/5/2 7/5/2 7/5/2 7/5/2 7/5/2 7/5/2 7/5/2 7/5/2 7/6/2 18: 21: : 3: 6: 9: 12: 15: 18: 21: : Time Figure 7 Measured vs. simulated tank charging and discharging processes for July 5, 22. verified by the measured data. This helps to refine the simulation models continuously. For the sample case operation described above, the predicted vs. the measured tank chilled water levels at each hour for the 24-hour period are show in Figure 7. The simulated process matched the actual process closely, especially for the discharging process. The charging process between midnight to 6 a.m. seems to be a little faster than the predicted process, which suggests that for this particular period, the campus load was slightly lower than predicted. The variation in the campus load from day to day could be caused by many factors other than the outside air temperature, such as the relative humidity of the outside air, building operation schedules, special events, etc. However, if a systematic offset is observed consistently between the simulated and the measured processes, it may suggest a permanent change in campus load has been introduced and the load prediction models need to be adjusted accordingly. CONCLUSIONS The chiller start stop optimization program developed for the four-price electricity rate structure saves operational costs by operating the chillers in the lower-price time periods. The concept is easy to grasp when the rate schedule is relatively simple. However, when the rate schedule gets more complicated, more sophisticated analysis is required for every individual scenario. Chiller start-stops based solely on operators experience and general judgment may not be adequate to make the best decision. The computer program automated the decision-making process for daily chillers scheduling. This makes sure the chillers are always operated under a near-optimum scenario for different load conditions. Of course, the program can never truly optimize the system s operation for every single day, simply because the model cannot predict the campus s real load with 1% accuracy 24 hours ahead. However, the program does have the flexibility to be easily adjusted to reflect the changes in campus s load pattern. During year 22 when the CSSO program was implemented, the program generally ran smoothly without major interruption. The facility took full advantage of the thermal storage system and the four-price electricity rate schedule to achieve an annual operational cost saving of $78,8 in year 22 (compared to the estimated utility cost if an equivalent flat rate schedule had been adopted). REFERENCES ASHRAE ASHRAE handbook HVAC applications, Chapter 34. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Fiorino, D.P Case study of a large, naturally stratified, chilled-water thermal energy storage system. ASHRAE Transactions 97 (2): Liu, M Rehabilitating a thermal energy storage system through commissioning. ASHRAE Transactions 15 (2): Shavit, G Operation and control of energy storage systems. ASHRAE Transactions 91 (1B): Tamblyn, R.T Control concepts for thermal storage. ASHRAE Transactions 91 (1B): Wei, G. 22. Practical optimization of full thermal storage systems operation. ASHRAE Transactions 18 (2):

12 APPENDIX A Tank capacity C tnk and chiller capacity C chlr CSSO Flow Chart Input next day's high/ low OAT Generate hourly OAT profile Calculate hourly and total campus cooling load L c Total predicted chiller load L p = L c - L t Determine tank charging start time t cs Determine tank discharging start time t ds Average chiller load R p = L p / H chg Total chiller runtim H tot = L p /R p Symbols L c = predicted campus cooling load L t = remaining tank capacity at tcs H led = Lead chiller runtime H lag = lag chiller runtime H bak = backup chiller runtime H chg = length of the tank charging period R p > 2 x C chlr N R p > C chlr N Y Y H led = H lag = H chg H bak =H tot -2 x H chg H led = H chg H lag = H tot - H chg H bak = H led = L p / C chlr H lag = H bak = Rp > 2 x Cchlr N Y Lead Start = t cs Lead stop = t ds Lag Start = t cs Lag Start = t ds Backup Start = t cs Backup Stop = t cs + H bak Lead Start = t cs Lead Stop = t ds Lag start = t ds - H lag Lag Stop = t ds Lead Start = t cs Lead Stop = t ds Lag Start = t cs Lag Stop = t cs + H lag - (t ds - 8) Backup Start = 8 a.m. Back Stop = t ds Lead Start = t cs Lead Stop = t cs + H led