FOCUS seeks to develop two distinct technology options to deliver low-cost, high-efficiency solar energy on demand, specifically: (1) new hybrid solar converter technologies to provide both heat, which can be stored at low-cost for later use, and electricity; and (2) new hybrid storage technologies that can leverage the simultaneous availability of both solar electricity and heat to dispatch solar electricity whenever needed.

Schematics of technology areas addressed by the FOCUS FOA. a) Hybrid solar converter (award Categories 1A and 1B). b) Hybrid storage system (award Category 2). The term “dispatchable” indicates availability as needed. Source: ARPA-E. Click to enlarge.

Despite the technology and deployment advances made by solar photovoltaics (PV), the current high-cost electricity storage options will likely limit PV penetration to less than about 5% of US primary energy before significant PV curtailments will be needed at times of high solar availability, ARPA-E said. Concentrating solar power (CSP), solar heating and solar hot water applications combined contribute less than 0.1% of US primary energy, and the deployment of these technologies is growing only slowly.

The high cost of grid electricity storage makes it best suited for high-value, short-term, frequency regulation; the lower value market for peak-shifted photovoltaic electricity is largely unprofitable, except where geography provides suitable reservoirs for lower-cost pumped-storage hydroelectricity.

Penetration of PV will ultimately be limited unless breakthrough technologies enable hours of electricity generated by PV to be cost-effectively stored. Models of the solar-resource-rich California electricity grid show that PV curtailments will begin at penetrations as low as 12%, while about one-third of PV electricity would be curtailed at penetrations of ~28%. The marginal economic value of installing PV is predicted to fall by half when PV meets more than 20% of the California utility load.

The current state of the German electrical grid provides a glimpse into the likely future of grid-parity PV in the United States. In Germany, about 5% of annual electrical energy production is now from solar resources, and solar energy frequently contributes more than 20% of the required grid power on a summer day. This level of solar penetration is enabled only through a combination of demand-side management, trading of surplus power at low cost to neighboring jurisdictions, and dispatch of expensive natural gas peaking or load-following plants.

Higher penetration of solar energy would require either slowdowns of generation from baseload plants, a function that they are not designed to perform efficiently, or dumping of surplus solar electricity. A recent analysis finds that a new German program offering up to 660 EUR/kW subsidy for storage tied to PV will not lower battery payback periods enough to induce new investment. Germany’s solar incentives now require PV systems to have a curtailment capability, to allow shut-off during periods of grid instability.

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Although CSP can integrate inexpensive thermal energy storage, the cost of CSP solar energy capture is high. Further, the scale of capital investment associated with CSP is a significant barrier to iteration of designs and rapid innovation.

In short, ARPA-E suggests, current PV and CSP solutions alone cannot provide the combination of low levelized cost of electricity (LCOE) and dispatchable output that will be required to enable large-scale solar energy utilization outside daytime hours.

The objective of the FOCUS program is to enable the development of these new technologies that, at maturity, will enable systems cost-competitive with other solutions: for example, electricity projects at utility scale will make dispatchable solar electricity competitive with conventional generation.

A subsidiary objective of the FOCUS Program is to form a diverse research community (including, e.g., CSP mechanical engineers, PV semiconductor materials and device scientists, optics/photonics experts, chemists, low-cost manufacturing experts and system integrators) who will innovate together.

Focus has three technical categories of interest:

• Technical Category of Interest 1 is the development of critical technologies needed for high sunlight-to-exergy efficiency hybrid solar converters that generate both heat and electricity. Category 1A covers the development of advanced hybrid solar converter technologies and prototypes. Category 1B addresses the development of novel topping devices that can efficiently convert solar energy directly to electricity at operating temperatures above 400 °C. These high-temperature topping devices will enable a second generation of hybrid solar converters.

• Technical Category of Interest 2 is development of hybrid storage systems for heat and electricity and the critical enabling technologies. These hybrid storage systems must generate electricity at higher round-trip exergy efficiency than is possible today using side-by-side combinations of purely electrical and purely thermal energy storage systems. The cost of a mature hybrid storage system must be low enough to permit topping electricity or PV electricity from the grid to be economically stored for later dispatch.

Category of interest 1A. Category 1A seeks critical technology improvements to enable hybrid solar converters in which the highest temperature of the thermal energy collection is between 150°C and 600°C. This range provides a disruptive change to the state-of-the-art, yet avoids the complications and expense of materials for T_h > 600°C. Utility-scale electricity generation is of particular interest to ARPA-E.

Applicants should propose designs for durable hybrid solar converters that integrate optics, topping devices and thermal receivers, with an optimized balance between heat and topping electricity production, considering both the impacts on system costs and the proposed system application. ARPA-E is interested in proposals that incorporate one or more of the following technological advances to maximize hybrid solar converter exergy efficiency:

• Hybrid solar converters in which a PV topping device is used at an intermediate temperature of a thermal energy collection loop that has a peak T_h incompatible with the efficient use of PV.

• Integrated spectrum-splitting approaches that place a spectrum-sensitive topping device on a secondary reflector that is in contact with the thermal collection loop. The secondary reflector would divert certain wavelength photons (e.g., IR or UV for PV topping) to a higher T part of the thermal collection system.

• Spectrally-selective thermal fluids flowing in front of the topping device to selectively absorb photons otherwise poorly utilized in the topping device, including fluids working through nanoparticle absorption.

• Hybrid solar converters that use a topping device other than PV cells, either alone or in conjunction with PV at lower T, to raise the system efficiency.

• High-efficiency single-crystal PV cells that are lifted-off reusable substrates for use as topping devices at high T. Such PV cells must be designed and tested for 25-year reliability under high T cycling or else be extremely inexpensive and incorporated in a converter that allows them to be economically replaced before they degrade.

• Efficient heat extraction from topping devices into the thermal collection medium with minimal loss of exergy.

• Topping devices that are highly efficient within a particular spectral band and are integrated with a hybrid solar converter optical design to selectively direct those wavelengths to the topping devices.

• Optics that economically collect and convert diffuse sunlight to electricity while also concentrating direct sunlight to a hybrid topping converter.

Category of interest 1B. Category 1B seeks prototypes of advanced technology concepts for efficient high temperature topping devices that can produce electricity from incident solar energy while achieving high effectiveness in transferring the waste heat to a thermal fluid at T≥400 °C. The devices should operate under solar concentration and eventually be manufacturable at costs compatible with a hybrid solar converter that produces exergy at below $1/W.

ARPA-E is interested in proposals that incorporate one of the following technological advances:

• Innovation to enable efficient PV in this temperature range;

• Devices exploiting photothermionic emission or other high temperature effects to convert sunlight to electricity; and/or

• Novel approaches to high T solar conversion.

Category of interest 2. This area seeks innovative systems that co-store heat and electricity and later output electricity, while demonstrating the performance of critical enabling components for the system. These systems must provide high round-trip exergy-to-electricity efficiency with the cost of storage below the cost available from a combination of today’s TES/turbine systems and advanced grid-scale electricity storage.

ARPA-E desires the hybrid system be technologically distinct from pure solar heat storage/generation with a heat engine, and also distinct from pure solar electricity storage (e.g., batteries or flywheels). The most useful systems will be flexible enough to accept a range of heat-to-electricity ratios of the exergy input.

ARPA-E is interested in proposals for storage systems able to accept solar heat input, including systems that may share some elements with today’s CSP plants (e.g., inexpensive molten salt storage and/or the use of a steam Rankine turbine). In this case, novel technology elements must be added to efficiently store electricity at the same time. ARPA-E is also interested in hybrid storage systems with non-turbine-based but cost-competitive methods of electricity generation.

Technologies of interest include:

• Thermomechanical approaches for mechanical storage with advanced heat management.

• Thermomechanical approaches utilizing both cold storage and hot storage, to raise the system Carnot efficiency.

• Chemical approaches that make energetic molecules by alternating thermal and electrochemical reactions.

• Chemical approaches that make energetic molecules by high-temperature endothermic electrochemistry.

• Storage systems incorporating solar photochemical inputs to reactions driven mainly by heat and electricity.

• Chemical approaches that make non-gaseous molecules, for ease of storage.