Nuclear Batteries

1. Nuclear Batteries NE 402 Fall 2013

2. Nuclear Battery • What is a nuclear battery? • Uses nuclear energy to generate electricity • How is this different from a nuclear reactor? • Nuclear power uses neutron-induced fission to generate electricity • Nuclear batteries utilize radioactive decay to generate energy • Why would we need this? • Nuclear plants are large, expensive, and generate a lot of radiation • Nuclear batteries are compact, long lasting, and generate minimal radiation • Nuclear batteries are useful in environments where conventional batteries are not suitable (size, power density, and battery lifetime are major factors)

3. Nuclear Battery • What are the types of nuclear batteries? • Direct charge collection • Indirect (scintillation) and PIDEC (Photon Intermediate Direct Energy Conversion) • Betavoltaic and alphavoltaic • Thermoelectric • Thermionic • Thermophotovoltaic

4. Nuclear Battery • What are the general factors to take into account? • Radioisotope selection • Alpha or beta; usually no gammas • Battery type helps choose which of the two types to use • Must take into account entire decay chain • Q-value of reaction • Density of the material • Half life • Short gives high energy density • Long gives long battery life

5. Nuclear Battery • Example beta emitters • Si-32, Cl-36, Ni-63, Sr-90, Pb-210 • Example alpha emitters • Po-210, Pu-238, Cm-242, Cm-244 • How do we calculate energy density? • The intrinsic energy density is found by • The energy EQ is dependent on the battery type • Takes into account efficiency factors (transport, conversion, transducing)

6. Direct charge collection • Emitted radiation particles in radioactive decay are charged and direct collection of them can be used to harvest energy • Several designs have been tested • General design is to collect charge on a metal plate acting as a cathode and the source acts as an anode • This constitutes charging a capacitor in-between the “plates” and discharging it through a “load”

7. Direct charge collection • Efficiency of this type of battery is a function of • Geometrical transport efficiency • Charging efficiency of collection plate • Discharging of collector through means other than the work load

8. Indirect (Scintillation) • Energy deposited is converted into light that is collected for energy production through a photovoltaic • Efficiency is dependent on scintillation efficiency, transport efficiency, and quantum efficiency of photo- voltaic

9. What is a Photovoltaic?

11. PIDEC (Photon Intermediate Direct Energy Conversion) • Energy deposited causes excimer formation in a gas (e.g. Xenon) that de-excites via photon emission • Energy is harvested through a photovoltaic • Efficiency is dependent on scintillation efficiency, transport efficiency, and quantum efficiency of photovoltaic

12. PIDEC (Photon Intermediate Direct Energy Conversion)

13. Thermoelectric Generators (RTGs) • Converts heat directly into electricity • This process is done using the Seebeck Effect • Voltage gradient created from charge separation • When J=0, the voltage gradient equals the electromotive force (see thermocouple) • If one side is connected to another material that has an opposite polarization and the other side through a work load then current is generated to thermalize the system. • semiconductors are common for power production because they can be doped to add extra carriers at higher temperatures • Typical efficiencies are on the order of 3-7 percent

14. Thermoelectric Generators General Purpose Heat Source

15. Multi-Mission RTG

16. Thermoelectric Generators (RTGs) • New designs are focused toward developing the Advanced Stirling Radioisotope Generator (ASRG) • A Stirlingengine drives two pistons to rotate a fly wheel to produce energy • http://www.explainthatstuff.com/how-stirling-engines-work.html • Its efficiency is expected to be up to four times larger than RTG (12-20 percent)

17. Thermophotovoltaic • A heated surface emits blackbody radiation and collecting this energy through a photovoltaic • Theoretically slightly higher efficiency than RTGs • Can couple this with RTG technology to create a hybrid system that will have an overall higher efficiency • Main challenge in this system creating a photovoltaic that has a high quantum efficiency at all wavelengths emitted with an appreciable probability

18. Beta and Alpha Voltaics • In most other systems the energy of radioactive decay was used in a thermal manner to generate energy • In this system the interactions of radiation with matter that create free charges is used to generate electricity • Here we want to minimize heat generation!!! • This is similar to direct charge collection and the indirect collection systems but • In indirect systems we are removing the middle man • In the direct charge collection systems we are doing direct energy collection, not conversion, and efficiencies are very low

19. Beta and Alpha Voltaics • In previous research single p-n junctions have been utilized • The question is, what is the maximum efficiency that can be achieved? • This question is answered by finding the total amount of energy released and comparing it to the amount of energy deposited within the p-n junction and the conversion efficiency within it

20. Alphavoltaic Theoretical Work • Published research results have indicated efficiencies of linearly graded single junction transducers of upwards of 20%. Let us check this result…

21. AlphavoltaicComputational Investigation • Simulation parameters • Spherical and slab geometries • Use silicon carbide because it is a wide band gap semiconductor and is found to radiation hard • Assume a one micron thick depletion region • Simulate energy deposition in spacing of one micron through the silicon carbide • Isotropic and mono-directional Po-210 alpha source of energy 5.307 MeV • Simulated in SRIM, GEANT4, and MCNPX

22. Alphavoltaic Theoretical Work

23. Alphavoltaic Theoretical Work

24. Alphavoltaic Theoretical Work Alpha particle energy deposition vs. distance in the slab model using SRIM/TRIM and in the slab and sphere models using GEANT4 for the mono-directional condition.

25. Alphavoltaic Theoretical Work Alpha particle energy deposition vs. distance in the slab model using GEANT4 and MCNPX for the isotropic point source.

26. Alphavoltaic Theoretical Work • In the previous slides we were shown the energy deposition curves and the corresponding numerical data • The energy deposited in any given region is not the efficiency of the nuclear battery • Many factors must be taken into account • Transport efficiency • Fill Factor • Driving potential efficiency • e-h pair conversion efficiency

27. Transport Efficiency • The transport efficiency is the fraction of energy deposited in the depletion region to the total energy of the alpha particle • Below the total power released by the source, Ptot, is equal to the activity of the source, A, and the energy of the alpha particle Eα • The short circuit current is defined through the equation below, where W is the energy required to create and electron hole pair and ηd is the transport efficiency found in the simulations and displayed in the tables

28. Fill Factor and Driving Potential Efficiency • Below is the equation that describe the open circuit voltage, where Eg is the band gap energy of SiC, e is the unit electric charge, and ηdp is the factor relating open circuit voltage to band-gap which is called the driving potential efficiency. • Below is the equation defining the fill factor from previously defined values and the maximum power attainable from the alpha voltaic cell, Pmax.

29. Fill Factor

30. Pair Production Efficiency • Not all of the energy deposited in the depletion region generates e-h pairs and this is described through the energy required to create an e-h pair to the band gap energy and is called the “W” value • Therefore the pair production efficiency can be described as

31. Total Efficiency • The total efficiency of any system is defined as the ratio of the input to output power • Using the previously defined equations we can define the total efficiency as • Substituting previous definitions into the above equation we obtain the total efficiency

32. Calculation of Total Efficiency • From the simulations we found that the transport efficiency for the • Mono-energetic source is 9.81 percent • Isotropic source is 17.2 percent • The pair production efficiency for SiC as been experimentally found to be 42 percent • The driving potential efficiency is variable but is generally 50 percent

33. Total efficiency • Using the values provided we find that the total efficiency is • 2.1 percent for the mono-directional sources • 3.6 percent for the isotropic sources • The Fill Factor here has been taken to be 1 but in typical commercial photovoltaic devices this is equal to approximately 0.8 • This means that this describes a theoretical maximum efficiency • So is 20% efficient claims in published literature correct?

34. Optimization • The controllable efficiency parameter is the transport efficiency • This is due to the width of the depletion region • The depletion region is dependent on the doping concentrations • Other methods to increase this factor include exotic boundaries (rings, textured) and multiple junctions

36. Other factors • Could use p-i-n structure • Could use intrinsic system with a driving potential • Did not take into account self-absorption in the simulations • Infinitely thin provides no self-absorption but large transducing area • Very thick provides smaller area but lower transport efficiency • So what is the most efficient?

37. Mass Minimization • Assume an intrinsic diamond transducer • No dead layer • 10% energy loss from driving potential • Simulations • Planar sandwich geometry • 10 Watt goal • 75 g of Pu-238 • 67 g of diamond • 70 We/kg potential • ASRG goal is 20 We/kg!!!