This clean energy idea was inspired by Google's detailed study of a concentrated solar power plant using a Brayton cycle gas turbine.

Temperatures inside a solar power plant reach 1000 ºC or higher, comparable to those in furnaces, so why...

(Not familiar with concentrated solar plants? Please see pictures 1 - 1b, otherwise scroll down to continue reading)

Picture 1 - How a Concentrated Solar Power Plant (CSP) Works

Picture 1: Google's solar gas turbine works almost like most gas turbines: The compressor takes in air, compresses the air, except the air is not heated by burning fossil fuels, but instead the air is directed through absorbing tubes which are fitted inside a solar receiver and which are heated by concentrated solar rays. Next, the hot air drives the turbines of an generator, producing electricity.

Picture 1a - Assembled Gas Turbine and Solar Receiver Picture 1b - Cutaway View Solar Receiver
Diagram Google solar gas turbine
Click picture to enlarge
Cutaway view of Google solar receiver
Click picture to enlarge

The Electric Version Of The Google Solar Gas Turbine: A Crazy Idea?

If the temperature inside the solar receiver can reach temperatures comparable to those in furnaces, why not place the absorbing tubes inside a real furnace?

A crazy idea? Maybe, maybe not.

The absorber tubes in the solar receiver snugly fit in a WK10000 electric bogie-hearth furnace (picture 2) made by Thermconcept.com.

The inside dimensions of the electric furnace are: width 125 cm, height 125 cm, depth 700 cm.

The absorbing tubes are 400 cm long and their outer diameter is 3,2 cm.

Given the inside dimensions of the electric furnace, it should be possible to stack the absorber tubes in 10 rows: 8 rows of 10 absorber tubes and 2 rows of 8 absorber tubes (allowing 3,2 cm space between tubes for heating space, fitting).

This amounts to a total of 96 tubes. The absorber tubes assembly would be 61 cm wide, 61 cm high (picture 2a) and 400 cm deep (picture 2b).

At the sides (top, bottom, left, right) there should 32 cm of space left for fitting (picture 2a) and 150 cm at each end of the tubes assembly for the air inlet / outlet headers and the air plenum to the engine (picture 2b).

Picture 2 - WK10000 electric bogie-hearth furnace Picture 2a - Stacking the tubes
Picture Thermconcept bogie hearth electric furnace
WK10000 electric bogie-hearth furnace

Inside dimensions: 125 cm wide, 125 cm high, 700 cm deep, volume 10940 liters
Maximum operating temperature 1280 Celsius
Power 300 kW

Click picture for more details (screen shot from brochure)

Picture 2b - Flowchart Electric Gas Turbine

Flowchart electric gas turbineComparing picture 1 and 2b shows the only difference between the solar gas turbine and the electric gas turbine is the heat source.

Nothing else of the hardware needs to be replaced. E.g. one can still use the same type of gas turbine.

Energy Efficiency

The type of gas turbine is the same, the electric furnace can stack the same type and number of absorber tubes as the solar receiver, also the temperature inside the electric furnace is the same as the temperature inside the solar receiver.

This implies that:

1. The gas turbine takes in and compresses the same volume of air
2. This volume of air is heated over the same distance
3. This volume of air is heated to the same temperature
4. Therefor, this volume of air can drive the same type of generator

The gas turbine using an electric furnace as a heat source should be able to generate the same electric output as the same type of gas turbine using an solar receiver as a heat source: 890 kW! Equals 2,9 times the energy input (Coefficient of performance (COP) of 2,9)!!!

The electric furnace requires 300 kW of power, this would mean the "electric" gas turbine could generate a net output of 590 kW.

Using an electric furnace has other advantages that might increase the net output:

1. Unlike the solar receiver, there is very little heat leakage from the electric furnace (see also below this box)
2. There are no temperature swings due to clouding over, sun dawn, sun set. This reduces pressure drops and problems caused by thermally induced stresses on the tubes
3. There is an even temperature distribution throughout the electric furnace, tubes are evenly heated from all sides and over their full length
4. Custom design the hardware (furnace, turbines, generator etc.) Google used some off the shelf hardware in its studies

When the net output is concerned, there is something else to consider; the difference between the energy needed for heating (melting) and the energy needed for keeping the electric furnace and its load at the desired temperature.

As picture 2c shows (source), there is a large gap between the energy consumption for melting aluminum / copper and the energy consumption keeping the aluminum / copper molten (holding).

Picture 2c - Specifications of electric melting / holding furnaces made by Nabertherm
Specifications of electric melting and holding furnaces manufactured by Nabertherm
For instance, the KF 80 Nabertherm electric furnace consumes about 50 kW to melt 190 kg of copper at 1000°C. In order to keep it molten at 1000°C, it consumes 6 kW with the lid closed and 11 kW with the lid open. This is about 12 and 22 percent (consecutively) of the energy consumption when melting.
It is possible to reduce the energy consumption for holding the temperature further by insulating the electric furnace in the same way as a thermal energy storage tank. This kind of storage is quite efficient.

For instance, the thermal storage tank of the Andasol power station has an efficiency of 93-97%. Quote from Stanford University coursework paper:"A fully heated tank of molten salts allows for the power plant to operate at full capacity for 7.5 hours after the sun has set. Only 3-7% less electricity is generated when thermal energy is stored in the molten salt tanks and used to produce electricity later as compared to when the energy is used to generate electricity directly. In other words, the molten salt storage system has an efficiency of 93-97%." Source quote.

With the insulation in place, ventilation switched off and running full power (300 kW), a fully heated WK10000 electric bogie-hearth furnace (picture 2) probably will catch fire without the gas turbine's airflow absorbing heat.

By embedding the absorber tubes in molten silicon (picture 2d) energy efficiency very likely can be further increased.

Picture 2d - Absorbing tubes embedded in molten silicon
Brayton cycle gasturbine with its absorbing tubes / heat exchanger embedded in molten copper
The 92 absorber tubes are 400 cm long and have an outer diameter of 3,2 cm and a radius of 1,6 cm. Their combined volume is 3.217 cm3 (π x 1,6 x 1,6 x 400) or 3,2 m3. The 92 absorber tubes easily fit in a crucible with an inner volume of 4 m3 (100 cm high x 100 cm wide x 400 cm long). This means less than 1 m3 (0,8 m3) of molten copper (inner volume crucible minus combined volume absorber tubes) is necessary to embed the absorber tubes.

A KF-80 Nabertherm electric furnace consumes about 50 kW to melt 190 kg of copper


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References and Addendum

Google's Concentrated Solar Plant papers

Thermconcept WK10000 electric bogie-hearth furnace
http://www.thermconcept.com/fileadmin/dokumente/Furnaces_for_Ceramic_Glass_Solar_eng.pdf (page 24)

Picture 2c is a screen-shot of page 11 from the Nabertherm catalogue. You can download the catalogue here.

Explaining Number of Tubes and Diameter

Unfortunately neither of the two papers by Google specifies the number of absorbing tubes nor their inner diameter, so the number in the picture 3 was used and it should be accurate.

< Picture 3 - Assembly of Absorbing Tubes

The 96 absorbing tubes are assembled in 16 sections.

Each section consists of 6 absorbing tubes.

Each absorbing tube is 400 cm long and has an outer diameter of 3,2 cm.

Fortunately the air mass flow (4,828 kg/s - red arrow picture 3a), pressure ratio (976 kPa / 9,76 bar - blue arrow picture 3a) and the air temperature ( 298 K / 25 C - green arrow picture 3a) are known.

The density of 1 m³ and 25 C warm air is 1,1839 kg (source Wikipedia), the gas turbine's mass air flow is 4,828 kg/s or 4,08 m³/s (4,828 kg ÷ 1,1839 kg). The pressure ratio is 9,76 bar, this reduces the volume of the 4,08 m³/s mass air flow to 0,42 m³/s (4,08 m³/s ÷ 9,76 bar).

Using the calculator on http://www.1728.org/flowrate.htm, it takes a ± 73 cm diameter pipe to transport 0,42 m³, so 96 absorbing tubes with an inner diameter of 0,76 cm (see table below) can do the trick, and compared to the outer diameter of 3,2 cm, it also shows that 96 absorbing tubes is a realistic number.

Velocity Flow Rate Pipe Diameter Diameter Of Each Absorber Tube
1 m/s 0,42 m³/s 73,127 cm 0,7617 cm (73,127 ÷ 96 tubes)

Picture 3a - Detailed flowchart of Google's solar gas turbine
Detailed flowchart of Google's solar gas turbine

All the images (except for image 2a and 2b) are screen shots from pictures in the documents listed above. They were cropped and/or edited to illustrate the idea.