How does Terrajoule Technology work ?

Terrajoule harnesses heat energy from the sun to produce electricity, day and night.

  • Mirrors concentrate sunlight on pipes.
  • The concentrated solar energy boils water to make high pressure steam.
  • The steam is expanded in a steam piston engine, causing the engine’s crankshaft to rotate.
  • The crankshaft drives an electrical alternating-current (AC) generator to produce electrical power.
  • To match the timing of the solar energy supply (daylight) to the electricity demand, heat energy must be stored for later use.
  • To store the energy contained in steam for later use, steam is condensed by injecting it into water that is contained in an insulated steel pressure vessel, thus capturing the energy of the steam by heating and pressurizing the water.  When electricity is needed, the heat in the water boils part of the water, converting it back into steam.
  • The steam retrieved from the steel pressure vessel is then used to power a steam engine, which powers an electricity generator to provide power on demand.

For more information, see Unexpected Technology.

 Is the technology proven ?

The technology is proven, both in performance and durability, by decades of use of each of the three system elements.  Terrajoule’s implementation of a system based on those proven technologies is in progress.  Terrajoule has developed a 100 kW 24/7 field prototype that will be operational in December, 2013.

The three system elements and their basis of proof are as follows:

  • Solar concentrators consisting of parabolic troughs that track the sun.  Many examples of this technology have been in continuous use for over 30 years.  Terrajoule procures critical components such as precision curved mirrors and receiver tubes from world-class vendors such as Flabeg and Schott.
  • Energy storage is via pressurized saturated water contained in steel pressure vessels.  The physical structure and performance conditions are the same as for industrial steam accumulators, used for decades to provide industrial steam across a wide range of flow rate.
  • Conversion from thermal energy to power on demand is via steam piston engines based on the Skinner Universal Unaflow technology that was the prevalent form of marine and industrial power in the early 1900s.  Long term data proved that such engines are more reliable than industrial diesel engines.

For more information, see Unexpected Technology.

 Can I see a Terrajoule system in operation ?

Yes.  Contact Terrajoule to arrange a visit to our field demonstration and technology development site at a walnut and almond ranch near Waterford, in California’s San Joaquin Valley.  This system is a full-scale prototype of the production system now under development.  It is designed to generate 100 kW of power for irrigation, up to 24 hours per day.   Full operation will be demonstrable (weather permitting) from December 2013.

How does Terrajoule Technology differ from the available solar thermal technologies ?

Terrajoule technology is designed to provide distributed-scale power on demand, 24/7.

Other solar thermal technologies are designed to provide utility-scale baseload power.



Other Solar Thermal

Distributed scale:100 kW to 10 MW and more Utility scale:50 MW to 250 MW
Dispatchable power:Rapid control from 10% to 500% of average output power Baseload power:Fixed output power level while operating, set by turbine specification
Low cost storage from 2 to 48 hours via Terrajoule proprietary storage technology:  Pressurized saturated water that supports dispatchable power (output power variable on demand) Higher cost storage: Two-tank molten salt system that supports extended hours of operation at a single output power level (not dispatchable)
Thermal-to-electrical power conversion: Reciprocating steam piston engines driving an AC electrical generator Thermal-to-electrical power conversion: Steam turbines driving an AC electrical generator

For more information, see Unexpected Technology.

 How do the costs compare with the other renewable technologies ?


PV Solar Panels + Batteries

Capital cost per peak Watt at 20% capacity factor, not including storage:$1.50/Wpeak to $2/Wpeak, depending on installation size and local conditions. Capital cost per peak Watt at 20% capacity factor, not including storage:$1.00/Wpeak (utility scale) to $2/Wpeak, depending on installation size and local conditions.
Storage cost per kWh of net electrical output storage capacity, normalized for replacement over 25 year life:< $100/kWhelectrical Storage cost per kWh of net electrical output storage capacity, normalized for replacement over 25 year life:> $300/kWhelectrical
Charge/discharge cycle limit:None Charge/discharge cycle limit:Depends on battery technology
Capacity degradation during useful life:None Capacity degradation during useful life:Depends on battery technology
Toxic or hard-to-obtain materials:None Toxic or hard-to-obtain materials:Depends on battery technology
Energy storage configurable independent of power output:Yes Energy storage configurable independent of power output:Depends on battery technology

 What is the storage medium used ?

Terrajoule uses pressurized saturated water, in an operating range from approximately 4 bar to 18 bar absolute (approximately 40 psig to 250 psig).  The water is contained in insulated steel pressure vessels, certified as ASME Section VIII compliant.  Each vessel has a volume of approximately 10,000 gallons. The pressure vessels are shipped, installed and operated in standard 40-foot shipping containers.

Each vessel (in its container) stores enough thermal energy to enable the system to store and deliver approximately 600 kWh of net electrical energy.

Energy is stored in the storage medium by directly injecting steam into the vessel.  The steam condenses by direct contact with the water.  To retrieve energy the heat in the water flashes the top surface of the water back to steam.

 Are steam pressure vessels a safety hazard?

No, there is no safety hazard.  All pressure vessels and piping are certified compliant with USA ASME Section VIII or Section I as appropriate, or equivalent standards in other regions.

Note that in the early 1900s there were a number of accidents involving rupture of steam pressure vessels.  The ASME codes were evolved to prevent such incidents.  The codes have been stable for decades, with no incidents associated with compliant vessels.

Costs of PV solar panels and batteries keep coming down.  Will Terrajoule costs keep pace?

Yes, in fact the pace of Terrajoule cost reduction will exceed that of PV solar panels and batteries.  Both PV solar panels and batteries have already enjoyed the benefits of high volume production, whereas Terrajoule is at the beginning of its volume curve.

What storage value do Terrajoule power plants bring to utilities?

Terrajoule’s power is dispatchable on demand, unlike PV and unlike CSP with molten salt storage (like all large turbine systems, CSP cannot be powered up or down quickly or efficiently).  Further, Terrajoule power has outstanding capability for rapid ramp up and down, with ramp times measured in seconds rather than minutes or hours.

Therefore, Terrajoule offers several levels of benefit to utilities:

  • At the basic level, if a Terrajoule plant supplies behind-the-meter power to a customer, it can meet the customer’s variable demand 24/7.  This means that Terrajoule capacity can expand its penetration of a grid without the limits to penetration associated with intermittent PV solar panels.
  • At the next level, Terrajoule can respond to market price signals and dispatch power accordingly, to fill in the systemic demand-supply shortfalls that result from normal changes in demand, and that are exacerbated by high penetrations of intermittent solar and wind.
  • At the next level, Terrajoule has the potential to respond rapidly to direct signals from a utility.  Since Terrajoule systems (with customer agreement) can be controlled remotely, it is possible to aggregate large amounts of Terrajoule dispatchable power, and respond to utility requests to dispatch power not only when but also where it is needed on the grid.  This is high quality spinning reserve with a rapid ramp capability.
  • Finally, Terrajoule systems produce spinning power, generated by AC generators, not inverters, so it is clean and inherently well suited to reactive loads on the grid.

 What is the site selection criteria for Terrajoule projects?

Good sites will meet the following criteria:

  • The application should offer a good return on capital invested by the customer or financer, based on energy costs avoided by the system.  If the customer uses a substantial amount of diesel to generate electricity or to power pumps or machinery, odds are it’s an excellent application, off-grid or on-grid.  Diesel use of 2,000 hours per year is a good indicator.
  • The typical power demand should be in the range 100 kW to 20 MW.
  • There should be sufficient land for the solar concentrators.    The requirement varies with local climate conditions, but is in the range of 5 to 10 square meters of land per MWh per year of electricity production.
  • The land should be available for solar use, with a modest purchase or opportunity cost.  For example, the cost of prime agricultural land in the California Central Valley is NOT a major economic factor for the payback on a Terrajoule system.  On the other hand, the cost of urban land may be prohibitive (e.g. San Francisco).
  • The average annual Direct Normal Irradiance should be above 4.5 W/m2.  This is a guideline only.
  • Since a Terrajoule system can employ wet or dry cooling, there are no major water requirements.

 Can air cooling be used in Terrajoule Power plants ?

Yes, optionally.  If water is available and there is value to heating the water (such as pre-heating for a process steam boiler, or for domestic or industrial hot water supply), then air cooling is not required.  Terrajoule systems generate about 2 kWh of heat energy per kWh of electrical energy generated, so in many distributed applications this heat can be put to valuable use.

Isn’t steam engine technology inefficient?

Terrajoule steam engines are highly efficient over a wide operating range, with an average conversion efficiency of thermal to electrical energy approaching 30%.  This results in an overall system efficiency converting solar power to electricity in the region of 20%, which compares favorably to other solar systems.

There is sometimes a misconception that steam engines are inefficient because historically, different steam engines evolved for different purposes.  For example, steam locomotive engines were often less than 10% efficient.  These were usually non-condensing engines with atmospheric exhaust (the choo choo effect), optimized for power-to-weight ratio and cost.

Marine and distributed power generation steam engines were more efficient.  By 1914 many engines were well-documented performing routinely with efficiencies approaching 25%.  There were two competing approaches to engine efficiency: compound (multi-stage expansion) and Unaflow (a thermal management topology).  Terrajoule has simply combined both of these approaches to achieve 30% efficiency.

Large steam turbines (e.g. 200 MW) achieve efficiencies above 40%, but bear in mind that efficiency is but one of many cost factors.  The economies of small scale and higher volume yield costs for Terrajoule that are substantially lower than turbine-based “CSP” plants.

How do Terrajoule O&M costs compare?

Terrajoule forecasts O&M costs of less than 1% of system cost per annum.  There is routine maintenance (e.g. annual oil changes) that is less than comparable power diesel engines, due to the lack of surface-to-surface wear in a steam engine, the lower parts count, and the lack of combustion products to contaminate the engine.

Terrajoule O&M costs will tend to be slightly higher than those for PV solar panel systems of equivalent electricity production.  For both types of system, the cost to clean mirrors or panels can be significant, and depends on local conditions.

Is storage needed?  With net metering it doesn’t seem to matter.

Wherever the price of power reflects its cost, storage is needed to harness solar power economically.  Otherwise there is no practical way to match solar resource supply to the demand.

For any application that is off-grid, or where the grid itself it intermittent, and diesel is used extensively, storage is required.

However, in several parts of the world (e.g. California), forward-looking public policies have temporarily eliminated the need for storage, by legislating that the electric grid will serve as the battery for the solar panels.  This has stimulated enormous growth in solar panels, and created grid problems due to the amount of intermittent power.  Now the need for storage is clearly evident, resulting in legislation such as California AB 2514 that mandates that utilities add storage capacity.

For more information see Grid Storage and AB2514.

The capital needed to scale up new solar technologies has been prohibitive.  How is Terrajoule different?

All components of a Terrajoule system are produced within existing large supply chains, including mirrors, pressure vessels, piping, valves, containers, engine parts, controls and sensors.  There are no new materials or manufacturing processes, and there is no need to build new factories to meet multi-billion dollar demand.

This contrasts with many new renewable energy technologies where the development of new materials, equipment, manufacturing processes and manufacturing capacity must be financed.  Since the cost of such development and capacity must be amortized, there is a significant impact on the delivered cost of the energy system.  By utilizing innovation in architecture and controls, rather than materials and processes, Terrajoule avoids this problem.