STUDY LIMITATIONS AND FUTURE WORK

This analysis did not perform a complete assessment of the value of CSP with TES. In addition to its limited geographical scope, there are several limitations to the analysis that will be addressed in future studies, includ­ing WWSIS II. A primary limitation is related to sub-hourly operation and ramp rates due to both variability and uncertainty. The simulations were performed at an hourly level and did not consider additional ramping that would result at higher time resolution due to solar variability. Further­more, while flexibility reserves were held to account for solar forecast error, these reserves were not dispatched. This would further increase gen­erator ramp requirements. Finally, the high ramp rates that are observed in these hourly simulations impose no additional cost penalty. WWSIS II will include 5-minute dispatch and the impact of ramping costs. These simula­tions should aid in identifying cost impacts on conventional generators and possible mitigation through the use of CSP with TES.

This study also did not perform a detailed assessment of the ability of CSP to provide ancillary services, including regulation, flexibility, or spin­ning contingency reserve. In conventional power systems, these services incur a cost due to two factors. The first is the additional O&M and other operational costs associated with additional cycling and other plant opera­tions required to provide ancillary services. The second is the “opportunity cost” associated with holding plants at part load compared to operating at full output. We did not have sufficient data on the operational cost of CSP plants in providing ancillary services and how these costs compare to oth­er generators to evaluate them in this study. We did perform simulations in an attempt to calculate any reduction in production cost when providing ancillary services from CSP, and found the reduction in total system cost to be extremely small. However, the modeled system has very low costs in general for provision of ancillary services due to the assumed flexibility of many generator types in the database. Additional analysis will be required to further isolate benefits of CSP providing ancillary services.

Finally, this analysis considered a single CSP technology type: wet – cooled trough-type plants with a limited range of storage capacities. Fur­ther analysis is needed to evaluate the impact of multiple CSP technology types under a range of renewable penetration scenarios.

9.6 CONCLUSIONS

Implementation of CSP with TES in commercial production simulation and planning tools is an important component of valuing this technology. This study evaluated the operation of CSP with TES in two scenarios of renewable penetration in a test system based on two balancing areas in Colorado and Wyoming. Overall, we found that the simulated CSP plants were dispatched to avoid the highest-cost generation, generally shifting energy production to the morning and evening in non-summer months and shifting energy towards the end of the day in summer months. This minimized the overall system production cost by reducing use of the least – efficient gas generators or preferentially displacing combined cycle gen­eration over coal generation. The system also dispatches CSP during the periods of highest net load, resulting in a very high capacity value.

Overall, the addition of TES to CSP increases its value; however, the difference in value between plants with and without storage is highly de­pendent on both the cost of natural gas and the penetration of other renew­able sources, such as PV. At low penetration of renewables, the inherent coincidence of solar and price patterns means that CSP without storage (and PV) has relatively high value. Combined with a relatively low gas price of $4.1/MMBTU used in this study results in an incremental op­erational value of TES of about $6.6/MWh over a plant without TES (at low RE penetration). At higher RE penetration, this difference increases as the value of mid-day generation is reduced—in the high RE test system this difference in operational value grew to $16.7/MWh. In addition, the capacity value of CSP systems with TES remains high, further increasing the difference in value associated with TES.