UTILITY-SCALE SOLAR LOAD CONTROL. Richard Perez, ASRC Christy Herig, NREL Ruth Mac Dougall, SMUD Bruce Vincent, SMUD

Transcription

1 UTILITY-SCALE SOLAR LOAD CONTROL Richard Perez, ASRC Christy Herig, NREL Ruth Mac Dougall, SMUD Bruce Vincent, SMUD 1. INTRODUCTION The objective of Solar Load Control (SLC) is to maximize the effective capacity of noncontrollable renewable power sources such as photovoltaics (PV). SLC acts by making up for any output deficit beyond a given load threshold with load shedding (fig.1). For power sources such as PV, which may have a naturally high correlation with loads (in particular loads driven by commercial A/C) the amount of SLC required to insure full PV availability is small [1,2]. Solar load control was initially devised to function with user-sited PV installations to maximize demand reduction. An efficient implementation for user-sited SLC is indoor cooling load management via end-use thermostatic adjustment (fig. 2). Figure 1: In the ideal situation at left, Peak load is reduced by the full installed PV capacity. PV output may be reduced before end of peak because of clouds and or evening, reducing the achievable peak load reduction (middle). By shedding load in response to PV output and load requirements the SLC restores the full load reduction (right). 1

2 Figure 2: For user-sited PV systems SLC can be implemented by acting on end-use settings (e.g., cooling temperature set points) in reaction to load and PV output signals. Utility-scale SLC: when considering an entire utility or a distribution system, the temperature-based end-use load control approach, although very effective in theory, would initially be impractical to implement. There exists, however, an effective utilityscale SLC implementation via direct load control (DLC) enhancement. The aggregate of direct load controlled customers constitute a reliable and dispatchable capacity reserve for a utility. Combining SLC action with DLC implementation could: (1) increase the firm capacity reserve margin of an existing DLC base and (2) decrease the probability that any given DLC customer may be activated --this may in turn attract more customers to the DLC pool, further increasing available capacity reserve. 2. OBJECTIVE OF THIS CASE STUDY The objective of this study is to quantify SLC requirements for SMUD by answering the question: how much load control would be needed to insure that PV + SLC meet all top loads beyond a threshold corresponding to the installed PV capacity 1? 3. METHODOLOGY 3. 1 Experimental Data SLC requirements are derived from time-coincident load and PV output data. 1 E.g., if the installed PV capacity is 10MW, the objective is for PV + SLC to meet 100% of the loads above a threshold equal to the utility peak minus 10 MW. 2

3 PV output data: PV output for the Sacramento area is inferred from time/site specific insolation data derived from satellite remote sensing. The accuracy of this methodology has been thoroughly evaluated [3]. Four years of data are available and were used for the analysis: 1998 through A nominal PV configuration optimized for mid-afternoon production was selected: 30 degrees-tilt, 40 degrees azimuth-west arrays. PV rating used throughout the report is PTC (AC output at 25 degrees ambient). Load data: Hourly SMUD system load data from 1996 to 2001 were made available. However, only the data coincident with PV output data from 1998 onwards have been analyzed. 3.2 PV penetration Six levels of PV penetration, respectively 2%, 5%, 10%, 15%, 20% and 25% of yearly peak load, were considered. 3.3 Quantifying solar load control The following metrics were used to gauge SLC performance 2 : 1. PV-only peak load reduction (without load control) 2. DLC capacity requirements, with and without PV (quantified in % of peak load reduction objective) 3. Maximum one-day DLC customer taxing with, and without PV (quantified in installed PV capacity-hours) 4. Total yearly cumulative DLC customer taxing with, and without PV (also quantified in installed PV capacity hours) 4. RESULTS Solar load control metrics are summarized in Table 1. (Temperature-based metrics are provided in Appendix) For illustrative purposes figure 3 shows the load, PV output and load control threshold for each year s peak load day, assuming a PV penetration of 10%. 2 For information, metrics based upon end-use temperature control are also investigated see appendix -- although, implementing this type of control may be impractical initially because it would require a widespread deployment of remotely controllable thermostats. These metric include: 1. Maximum 1-hour end-use cooling temperature mitigation with, and without PV (oc) 2. Maximum daily end-use cooling temperature mitigation with, and w/o PV(oC-hours) 3. Total yearly end-use cooling temperature mitigation with, and without PV(oC-hours) 3

4 TABLE 1 Photovoltaic and Solar Load Control Metrics For Grid-Penetration ranging from 2% to 25% with PV without PV % peak load reduction corresponding PV-size (MW-ptc) PV Load reduction without load control (MW) Maximum instantaneous SLC capacity draw (% installed PV capacity -- column 2) maximum daily SLC energy requirements ( installed PV - hours) total SLC energy requirements ( installed PV - hours) Maximum instantaneous SLC capacity draw (% installed PV capacity -- column 2) maximum daily SLC energy requirements ( installed PV - hours) total SLC energy requirements ( installed PV - hours) % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % %

5 Figure 3: Load and PV output on each year s peak day Note 1: PV output corresponds to 10% grid penetration Note 2: The difference between the threshold (red line) and the Load-PV line represents the required Solar Load Control action. The Load PV line with SLC action would be capped at the red line. 5

6 5. DISCUSSION Peak load reduction without load control: The peak load reduction achieved by PV alone without control ranges from 60-65% of installed PV capacity at low grid penetration (2%), down to ~35% at high penetration (25%). Results are fairly consistent from year to year. DLC capacity requirements: The DLC capacity that would be needed to achieve a load reduction equal to the installed PV capacity, ranges from ~35% of installed PV capacity at low penetration, up to ~ 65% at high penetration. Taking a specific case for illustrative purposes: 10% PV penetration in 2001 corresponds to an installed PV capacity of 145 MW; the required DLC capacity to guaranty that all loads above this penetration threshold are met would be 42% of 145 MW, that is 61 MW. In other words, an existing DLC capacity of 61 MW would see its effective capacity reserve more than doubled to 145 MW by working in tandem with PV. Maximum one-day DLC energy requirements: The maximum one-day energy requirements from a pool of DLC customers amounts ~0.5 hour worth of installed PV capacity at low PV penetration, up to ~2.5 hours at high PV penetration. By contrast achieving the same level of load reduction without PV would respectively require from 2 to 8 hours depending upon PV penetration i.e., the maximum one-day taxing of DLC customers would be reduced by a factor 3-4 when using PV in tandem with DLC. Taking the same illustrative example as above -- 10% penetration in The maximum one-day DLC requirements would be 1.3 hours * 145 MW = 189 MWh, with a maximum instantaneous DLC draw of 61 MW. Without PV, the maximum one-day DLC requirements would be 4.1 hours * 145 MW = 595 MWh, with a maximum instantaneous DLC draw of 145 MW. Total yearly DLC energy requirements: As shown in Table 1, at low PV penetration, guarantying 100% PV capacity for the SMUD grid would require a total DLC energy burden of roughly one installed-pvcapacity-hours. At 25% penetration, the DLC burden reaches about 14 installed-pvcapacity-hours. Still, this is 5-7 time less that the DLC burden that would be required to meet the 25% top load without PV. Again, taking 10% penetration in 2001 as an example, the yearly DLC energy requirements would be 1.6 hours * 242 MW = 387 MWh, with a maximum instantaneous DLC draw of 99 MW. Without PV, the yearly DLC requirements would be 14.1 hours * 242 MW = 3,412 MWh, with a maximum instantaneous DLC draw of 242 MW. Therefore, making reference to the 300 MW DLC capacity currently available to SMUD dispatchers, assigning one third of this reserve to work in tandem with an installed PV base of 242 MW, would guarantee 242 MW of peak load reduction. The fraction of DLC 6

7 customers assigned to PV would be taxed nine times less than if this fraction had been assigned to accomplish the same load reduction without PV. 6. CONCLUDING REMARKS This analysis shows that, for SMUD, operating PV in tandem with an existing Direct Load Control capability could stretch the dispatchable capacity of the DLC pool in two ways: (1) ~ doubling the DLC instantaneous dispatchable capacity and (2) achieving this increase with considerably less cumulative impact on the DLC d customers this more benign impact on customers could attract more customers to the DLC pool, further increasing instantaneous dispatchable capacity. Operational effectiveness could be further increased by considering deployment of customer setting control in addition to DLC (e.g., cooling load mitigation see appendix) or by considering micro-storage deployment. The next logical steps of this analysis are: (1) addressing the logistics of managing a fraction of the available DLC base in tandem with the installed PV capacity at SMUD; (2) testing the implementation of PV + DLC; and (3) developing appropriate financial arrangements for the fraction of DLC customers assigned to Solar Load Control. REFERENCE 1. Perez, R., J. Schlemmer, B. Bailey and K. Elsholtz, (2000): The Solar Load Controller -- End-use maximization of PV s peak shaving capability. ASES-2000 Conference, Madison, WI 2. Perez, R., S. Letendre and C. Herig, (2001): PV and grid reliability: availability of PV power during capacity shortfalls. Proc. ASES Annual Meeting, Forum 2001, Washington, DC. 3. Perez, R. M. Kmiecik, D. Renné and C. Herig, (2001): Remote Monitoring of PV Performance Using Geostationary Satellites, Solar Energy 71, 4,

8 APPENDIX Solar Load Control Metrics Quantified in terms of utility-wide cooling load mitigation For PV-penetration ranging from 2% to 25% with PV without PV % peak load reduction corresponding PV-size (MW-ptc) PV Load reduction without load control (MW) Maximum 1-hour temperature offset (degree-c) slc maximum-1-day-degree-hours Total degree hours for year Maximum 1-hour temperature offset (degree-c) slc maximum-1-day-degree-hours Total degree hours for year % % % % % % % % % % % % % % % % % % % % % % % %