Nuclear Batteries NE 402 Fall 2013
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)
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
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
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)
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”
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
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
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
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
Thermoelectric Generators General Purpose Heat Source
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)
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
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
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
Alphavoltaic Theoretical Work • Published research results have indicated efficiencies of linearly graded single junction transducers of upwards of 20%. Let us check this result…
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
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.
Alphavoltaic Theoretical Work Alpha particle energy deposition vs. distance in the slab model using GEANT4 and MCNPX for the isotropic point source.
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
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
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.
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
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
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
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?
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
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?
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!!!