Category Archives: featuredposts

Modular CSP

Modular CSP (M-CSP): Baseload, Dispatchable, Stable, and low cost (0.045 $/kWh), solar energy

What to do when the sun is not shining. Photovoltaics (PVs) plus batteries plus grid (Gas and coal turbines) backup are expensive. Concentrated solar power (CSP) is considered a “nearly obsolete” technology, yet it is the only technology that is based on a heat engine, that can replace fossil fuel heat engines. In a few consecutive days with poor solar radiation, the heat engine can be driven by green hydrogen or gas for 24/7/365 full dispatchable electricity. Today, CSP is an expansive utility scale power plant (0.09$/kWh) with reliability and financing challenges.

A modular-CSP (MCSP) is made of an array of small receivers where the heliostats shift from one receiver to the other along the day targeting the receiver next to the sun. This way the field efficiency is doubled, and no costly huge tower, resulting half the cost of a central CSP (0.045$/kWh). The modularity, also supports an exponentially growing market, as in PVs, with a potential for a global impact in a few decades. We work to demonstrate M-CSP in lab scale, analyze its performances along the year, and develop its control system for supporting baseload, dispatchable, stable, and low cost, solar energy. M-CSP requires a small and efficient external heat engine that doesn’t exist. See the next topic for our solution.

Modular CSP


Overcoming the thermodynamic efficiency limit on small external-heat engines for 24/7/365 solar electricity green Hydrogen, and waste heat

M-CSP requires a small (<1MW) and efficient (>40%) external-heat engine, not existing today due to thermodynamic considerations. Such an engine will also increase the green hydrogen production capacity factor, which will reduce its cost to the level of gray Hydrogen, and will allow to harvest waste heat.

In all heat engines that we know, only gases are used, and for two roles; i.) Perform the work by expansion and compression. ii.) Transfer the energy into the engine. This 2nd role is challenging for external heat sources, because gases carry negligible thermal energy per volume (volumetric heat capacity), and tend to behave as an ideal gas. This results in a non-isothermal expansion/compression, which is counterproductive since the temperature drop arrests the expansion and drastically reduces the efficiency.

Increasing energy density by three orders of magnitude and inducing isothermal gas expansion is what we solve by demonstrating the first dual-phase (HTL & gas bubbles) engine where the HTL supplies the heat along with the gas expansion.

In our engine, HTL such as molten salt flows in a nozzle with a thermal energy density that is orders of magnitude higher than gasses (per volume). Pressurized bubbles of air are injected into the nozzle and expand isothermally due to the negligible heat capacity of each bubble compared to its large surface area. The bubble’s expansion accelerates the HTL in the nozzle converting the heat and pressure into kinetic energy of the HTL. Converting the kinetic energy of fluids into electricity is extremely simple and achieved by hydroelectric-like technology, which has a tag price that is a fraction of the cost of steam turbines.

external-heat engines

battAIRy team

Congratulation to our MSc student Joseph (Joey) Cassell and his battAIRy team

for winning the 2nd place in the Hackathon on utility-scale energy storage.
The team evaluated liquid air technology for seasonal energy storage. Their solution used a solar powered thermoacoustic engine to liquify air at a round trip efficiency (from sun to liquid air and back to electricity) of 15%, comparable to a combined PV + pumped-hydro system. This cost effective approach allows for long-life, high energy density, reliability, and geographic flexibility not found in traditional utility scale energy storage technologies.

battAIRy team


Congratulation to PhD student Dror Miron and his Heat2Heat team

for wining the 1st place in the Hackathon on green energy utility scale storage.  The team showed how thermochemical storage Ca(OH)2/CaO that has more than 77% energy cycle efficiency (in retrieving heat at 500C) without the need for catalysator.

It is also cost effective, non-toxic, stored at room temperature, which is perfect for seasonal storage.  Together with our concept for M-CSP and a reversible reactor at the heat demand location (industrial consumers) this concept can solve 25% of the world global energy consumption.

Heat2Heat team

Generalization of Kirchhoff’s Law: The inherent relations between quantum efficiency and emissivity; ε=α(1-QE)

Kirchhoff’s LawPlanck’s law of thermal radiation depends on the temperature, T, and the emissivity, ε, which is the coupling of heat to radiation depending on both phonon-electron nonradiative-interactions and electron-photon radiative-interactions. In contrast, absorptivity, α, only depends on the electron-photon radiative-interactions. At thermodynamic equilibrium, nonradiative-interactions are balanced, resulting in Kirchhoff’s law of thermal radiation, ε=α. At non-equilibrium, Quantum efficiency (QE) describes the statistics of photon emission, which like emissivity depends on both radiative and nonradiative interactions. We theoretically and experimentally demonstrate a prime equation relating these properties in the form of:

ε=α(1-QE), which is reduced to Kirchhoff’s law at equilibrium.
Read more… “NATURE Phot – Generalized Kirchhoff Law

Ideal Light Source

Ideal Light Source – De-coherence without time averaging for high radiance uniform light source

Imaging of microscopic structures, photography, and excitation of materials benefit from a uniform and high-power light source. Since lasers are high radiance light sources, they are good candidates for these purposes. However, their high coherence results in speckles; a nonuniform intensity.

Spectral Radiance is the thermodynamic quantity describing illumination and is defined as the power of the radiation per wavelength per solid angle per area.

Looking on this definition we see the inherent tradeoff between high radiance and uniformity. High radiance source requires a narrow spectral and angular width, which results in high coherence and speckles.  Conventionally, speckles reduction is done by a time average of a fast moving element which is time-consuming (For example a rotating diffuser combined with a slow detector).

Our aim is to reduce the coherence instantaneously, thereby allowing fast and uniform illumination.

Our solution is taken from the diffusion process, described by a “Random walk” behavior. Consider an ensemble of random walkers starting at a point location and time. In time, they spread in a diffusive form. More important, looking at meeting events between different walkers, as time evolves the average time difference between walkers increases.  Assuming short memory (Drunk random walkers), after some time of evolution, the meetings occur between walkers that do not recognize each other. Going back to photonics, coherent light propagating in a diffused media acquires Optical Path Difference (OPD) that increases in time. After sufficient propagation, the average OPD exceeds the coherence length, which results in incoherent interaction and the elimination of speckles.

Specifically, we design a multi-mode fiber which contains pre-designed static scattering centers along its propagation axis. Propagation through the fiber volume, filled with these centers, results in different OPD acquired by different components of light fields. By engineering the centers, so that the standard deviation of the OPD is proportional to the coherence length of the light source, we achieve orders of magnitude reduction in the coherence of light, while maintaining its high radiance.


Luminescent Solar Power

The challenge in solar energy today is not the cost of photovoltaics (PVs) electricity generation, already competing with fossil fuel prices, but rather utility-scale energy storage costs. Alternatively, low-cost thermal energy storage (TES) exists but relies on expensive concentrated solar power (CSP). A technology, able to unify PV conversion and TES, may usher in the era of efficient base-load renewable power plants. Spectral splitting, where inefficient photons for PV conversion are redirected and thermally utilized, is economically limited by the low yield of each generator. Operating PVs at high temperatures while utilizing the thermalization induced heat for the thermal cycle is another possibility; yet, while conceptually supporting full utilization of solar energy, it is limited by PV efficiency reduction with temperature increase. My group recently introduced the concept of Luminescence Solar Power (LSP), where sunlight is absorbed in a photoluminescent (PL) absorber, followed by red-shifted PL emission matched to an adjacent PV band-edge. The PL absorber temperature rises due to thermalization, allowing spatial separation between heat and free-energy, for maximal harvesting of both. We solved the material challenge by demonstrating tailored PL with an efficiency of up to 90% while operating at 550oC. At such temperatures, LSP efficiency is 200% higher than conventional CSP and may lead to a reduction in the levelized cost of electricity (LCOE) to below 3¢/kWh

Conceptually, if PV efficiency would tolerate high temperatures, as high as 550°C, for example, it would be beneficial to concentrate solar radiation onto PV cells, harvesting the available free energy while in parallel harvesting high-quality thermal energy through a conventional steam cycle such as exists in CSP. Such a method can potentially surpass any spectral splitting method, where part of the solar spectrum is channeled to the PV while the other is channeled to a heat cycle which falls short by sacrificing heat utilization for PV efficiency or vice versa. Unfortunately, operating PVs at a high temperature cannot be done, as PV efficiency decreases sharply with temperature. Nevertheless, what cannot be done with electrons can be done all-optically. Recently, my group presented and experimentally demonstrated a new concept, Luminescence Solar Power (LSP), where solar radiation is focused onto a photoluminescence (PL) absorber having a high EQE while operating at 550°C. The PL has a narrow line shape emission that can match the band-edge absorption of single or dual-junction PVs, offering concentrated-PV (CPV) above 30% efficiency with minimal heating of the PV. The high-quality heat at the PL-absorber is collected by a heat transfer fluid (HTF) and converted into electricity at a turbine efficiency of 40%. The concept of using PL to separate free energy and high-temperature heat in this manner has never been explored before, even though each component of the system, namely the CPV cells, CSP, and the PL-absorber, rely on well-established technologies. Externally LSP and CSP installations appear the same. The figure above depicts the internal mechanism where the PL-absorber is placed at the focal point of a solar field similar to what is done in CSP. The light is absorbed and re-emitted towards a single or dual-junction PV cell at the back side. A preferred directional emission is achieved by  AR-HR coatings. HTF maintains the PL-absorber’s temperature at 550°C while transferring the heat to the turbine operating at 40% efficincy.

  • The outcome:
    • PV cell operates more efficient – high concentration & low temperature (direct electrical energy)
    • The absorber is heated – act as a thermal source for a steam turbine (stored energy)
  • Absorber material:
    • Rare-earth emitters (Nd3+, Yb3+…) → Narrow emission to match the PV band-edge, stable External quantum efficiency (EQE) at high temperatures.
    • Adding Cr3+ → enhanced solar absorption, efficient energy transfer to the rare-earth emitters.
    • YAG/Silica… → Transparent, thermally stable, efficient host for rare-earth emitters.
  • Experimental results:
    • High absorption up to 650 nm
    • EQE of 90% @ 600°C
  • Total device efficiency of above 40% with 30% storage is in reach.

Current status:

We started working on a heat engine that is soo cheap, allowing modular CSP without the need for PL crystals, see heat engine item.