March 15, 2017
SOLAR 2016 provided participants with high quality information on a variety of topics related to advances in solar energy. The Technical Review Committee offers the following abstracts as examples of some of the particularly cross-cutting, cutting edge work presented.
Novel Designs and Equipment Propel Solar Thermal’s Future
By Peter Skinner
Once the darling of sustainable living enthusiasts and commercial concerns, solar thermal systems have lost some momentum to grid connected PV systems even though PV’s energy efficiencies are a small fraction of the thermal type. To stay competitive, thermal systems must increase their efficiency and find applications for the thermal surpluses summer brings to many installations. These goals can be attained by system designs that are better adapted to a facility’s traditional loads and can fulfill other loads including space conditioning and PV panel cooling. Furthermore, thermal storage approaches that extend the storage over longer periods, that include phase change materials, that store and utilize heat at cooler temperatures, and that employ heat pumps can increase the utility and efficiency of advanced solar thermal systems. The availability of affordable high efficiency cold climate water to water and air to water heat pumps are the agents of change installers must become comfortable designing into new projects. Cutting edge solar and thermal energy management and transformation equipment and systems integration concepts can make solar thermal systems equally relevant in today’s PV dominated conversation. As the design and equipment for solar PV, thermal systems, and HVAC designs have matured and evolved, so too have the expectations for on-site energy production and occupant comfort at passive house and/or net-zero design bases buildings. The new netZero Village apartment complex near Schenectady, NY illustrates how professional design collaboration, dedicated and well guided tradespeople, proper choices and arrangements for shell materials and HVAC equipment, and properly sized solar PV and thermal systems can create an affordable multifamily complex that meets netZero energy goals. 65 KW of PV and 26 solar thermal systems coupled with highly efficient cold climate heat pumps and Passive House quality shell design has made this a sustainability standout. http://bit.ly/2gUmeEk.
Forecasting Solar Power and Irradiance – Lessons from Real-World Experiences
By Stephen Jascourt
MDA Information Systems, LLC developed a solar irradiance and power forecasting system a few years ago based on a first principles science foundation employing high-quality scientific datasets such as AERONET and SURFRAD and utilizing the REST2 clear sky model as an underlying basis for the full-sky forecast. A diverse multi-model ensemble of numerical weather prediction (NWP) forecasts are bias corrected, partitioned into direct beam and diffuse irradiance, projected onto solar panels of any desired orientation, and blended to generate an optimal forecast and a distribution of outcomes. The forecast system now ingests site observations of power from solar farms, performs quality control, and creates power forecasts using site-specific power curves generated from these data. The site data and satellite observations are used to improve the short-range forecast. Also, forecasts for aggregate distributed generation, such as for Germany, are produced using the same principles and tuned using publicly posted estimates of actual aggregate generation. While working with clients and client-supplied data, lessons were learned and new capabilities added, and these are highlighted here. Forecasts were made for challenging locations where daytime cumulus clouds and occasional storm systems passing through resulted in variability on time scales of minutes, hours, and days. We learned that forecasting minute-to-minute variability is important even to hourly average power due to inverter-capped power output. While minute-to-minute irradiance and power is not predictable, we do predict the magnitude of sub-hourly variability a day ahead. The quality of validation data is paramount. Ground-based measurements are used for calibrating NWP models, generating power curves to convert irradiance to power, and site measurements are used in the short-range forecast. Solar farm data and even irradiance observations at surface observing sites are beset by many data quality issues which we address. Solar monitoring instruments are typically not maintained or sited, resulting in calibration drift and shadowing, and stuck solar farm data loggers. For solar farms with tracking, knowing the tracking angles is essential to projecting the forecast irradiance onto the plane of array. However, every tracking solar farm for which we have analyzed data has the panels resting horizontally overnight and the transition between optimal mid-day tracking and the resting position is not documented. We infer it from careful calculation. We have found examples where some panels are not tracking, where morning ascent and evening descent are not symmetric, and where the peak tilt exceeds the specifications on the tracking equipment. Finally, we have also found the NWP models predict sunny days too often under certain types of meteorological conditions, and we correct for this.
Intra-hour solar forecasting and cloud height detection advances
by Jan Kleissl
Cloud base height (CBH) is a critical input to short-term solar forecasting algorithms. Yet CBH measurements are difficult to obtain. Existing methods to detect CBH include radiosondes, ceilometers, and the stereographic method. However, these methods are deficient for intra-hour forecasting due to high costs or low temporal resolution. We describe the integration of a cloud shadow speed sensor (CSS) with angular cloud speed from a sky imager to determine CBH. Furthermore, an improved methodology to determine cloud motion vectors from the CSS is presented. This method offers lower noise and greater accuracy and stability than existing methods. Two months at the UC San Diego campus were used for validation against measurements from meteorological aerodrome reports (METAR) and an on-site ceilometer. Typical daily root mean square differences (RMSD) are 126 m which corresponds to 16.9% of the observed CBH. Normalized RMSD remains below 30% for all days. The daily bias is usually less than 80 m which suggests that the method is robust and that most of the RMSD is driven by short-term random fluctuations in CBH. Unlike sky image stereography the present method can be applied to measurements at a single site making it widely applicable. Also recent advances in sky imager solar forecasting, specifically methods to obtain cloud optical depth, DNI, and diffuse irradiance from sky imagery are presented.
Past their Prime or Primed for Progress? The Role of State RPS Policies in Supporting U.S. Solar Market Growth
By Galen Barbose
State renewables portfolio standard (RPS) programs have been a key driver for the growth of solar energy in the United States. Although wind energy has historically been the primary source of new renewable capacity built to service RPS requirements, solar has recently come to the fore, reflecting both the proliferation of solar carve-outs and the growing economic competitiveness of solar vis-à-vis wind. As a result, RPS requirements represent almost 75% of all U.S. solar capacity additions through 2015, including roughly 5 GW of solar capacity for RPS solar carve-outs (mostly distributed PV in the Northeast and Mid-Atlantic regions) and an additional 11 GW currently being used to meet general, non-technology-specific RPS obligations (mostly utilityscale PV and CSP in the Southwest). Going forward, current RPS policies will require an additional 60 GW of renewable capacity by 2030, mostly in the Mid-Atlantic and West, much of which will likely be solar. However, a variety of uncertainties lie on the horizon that could curtail that growth. Legal and statutory challenges to RPS rules have been, and continue to be, raised in many states. RPS cost caps and declining alternative compliance payment schedules may constrain further growth. And many states are scheduled to reach their final RPS targets within the coming 5 to 10 years, with varying prospects for extension or expansion. This presentation discusses these and other trends, highlighting the past and future role of state RPS programs in supporting U.S. solar market growth.
The Right Tone Of VOS: Improving the Argument for Distributed Community Solar
By Jill Cliburn
The value of solar (VOS) argument, which establishes the utility value of solar in terms of avoided costs and benefits of capacity, energy, and aspects of transmission, distribution, compliance and risk management must be credited with raising solar from the status of a pesky “negative load” to that of a genuine and important resource. However, the VOS approach has evolved into an analytic specialty, increasingly complex and controversial, especially in cases where availability of lower-cost centralized solar offers utilities an alternative to building a business case for the higher first-costs and potentially greater rewards of distributed solar fleets. The authors have been in dialog with utilities around solar program design and pricing challenges for a decade, studying decision processes, exploring points of resistance to distributed strategies and testing ways to overcome them. We draw on that knowledge and especially on new experience leading the SunShot-funded Community Solar Value Project, which promotes utility acquisition (through PPA or direct ownership) of mostly commercial scale distributed projects for community solar programs. What is the most effective way to argue—even within the utility—for a distributed strategy, when there is an apparent pricing gap between programs based on distributed projects and those based on centralized solar? This presentation describes an alternative to an all-inclusive VOS approach to arguing for distributed solar versus centralized options. It starts from the back end of the analysis, establishing a target all inclusive price (rate plus an additional charges or credits and services) that the utility would need to offer customers. Then it estimates the gap between the sticker price of a distributed versus centralized option. The first step in filling the pricing gap draws on fairly accepted VOS-type values. But the second step involves values that are realized over a longer term or considered external to a typical rate-making process. These are mapped onto the appropriate utility internal stakeholders and their objectives. For example, these may include various distribution system values, which are real but hard to quantify, or strategic utility-business values, such as customer retention or values from co-marketed load management. Rather than pressing the limits of spreadsheet modeling, we focus on the power of a compelling narrative, supplemented with information from modeling. Thus, we emphasize building a business case to help utility decision-makers move into the future. The authors will lay out a 3-step process, document utility reactions, and provide examples where distributed community solar benefits could be expressed partly as they affect rates, partly as they affect “fixed” costs and partly as they are reflected in program credits or services. It also will touch on a next-step assessment of policy/regulatory considerations for widespread use.