Radioisotopic battery for long-life, buried autonomous power

Indirect conversion radioisotopic batteries (ICRBs) are investigated for use as long-life power source for autonomous buried applications. As part of this work the optimum configuration of this class of battery has been experimentally investigated. An ICRB was buried at a depth of 90cm for two months during which time its voltage was monitored, with these results presented. The ICRB successfully demonstrated the buried operation of this class of battery and suggested that the power of the device halves at twice the half-life of the radioisotope used.


Introduction
Radioisotopic batteries have been investigated before, however indirect conversion radioisotopic batteries (ICRBs) have received little notice. Much focus has been given to thermal cycle types of radioisotopic battery, such as those used by NASA [1], which require a large mass of radioisotope to effectively operate and are therefore unsuited to terrestrial use. The other types of radioisotopic batteries commonly reported either consist of charging a capacitor with the charged particles of radioactive decay [2] (which results in unusably high voltages, often 10kV or more) or involve generating electricity in a PV cell directly from the kinetic energy of the radioactive decay particles [3]. This latter type of radioisotopic battery produces radiation damage and therefore premature degradation of the PV cells, limiting the useful lifetime of the battery [4]. ICRBs do not suffer from these drawbacks.
ICRBs consist of a radioisotope in close proximity to a phosphor which generates photons when impacted by the decay particles of the radioisotope [5]. The photons travel to a PV cell where they are then converted into electrical energy in the conventional manner [6]. This energy may then be stored in a capacitor to be used when required. A schematic demonstrating this process is shown in figure 1.

Figure 1. Operating principle of an indirect conversion radioisotopic battery
Gaseous tritium light sources (GTLSs) consist of a glass ampule coated on the inner surface with a thin layer of phosphor and filled with pressurized tritium gas. GTLSs therefore comprise two of the processes necessary to produce an ICRB and were used during this research availability for use under UK law [7

Radioisotopic battery configuration selection
A preliminary investigation of the most effective configuration of ICRBs was undertaken using commercially available PV cells and GTLSs. other was investigated in terms of these ICRBs was measured indi capacitor at regular and timed intervals. The power stored) in the capacitor over the time interval between readings was then calculated through application of equation (1) (where previously measured voltage and system was used to measure the voltage of these ultra aluminium electrolytic capacitor. The investigated configurations of ICRBs are displayed in figure PV cell without reflective casing, (b) used two GTLSs on a PV cell with reflective casing, (c) used two GTLSs sandwiched between two PV cells surrounded by re surrounded by three PV cells and reflective casing, and (e) used two GTLSs surrounded by four PV cells and reflective casing. The results of generated against the voltage at which that power was produced. were of amorphous silicon type, chosen due to their superior performance at converting low intensity light into electricity.
Operating principle of an indirect conversion radioisotopic battery Gaseous tritium light sources (GTLSs) consist of a glass ampule coated on the inner surface with a thin layer of phosphor and filled with pressurized tritium gas. GTLSs therefore comprise two of the processes necessary to produce an ICRB and were used during this research due to their sturdiness and lability for use under UK law [7].
attery configuration selection A preliminary investigation of the most effective configuration of ICRBs was undertaken using commercially available PV cells and GTLSs. The layout of the PV cells and GTLSs relative to each other was investigated in terms of the power generated by the manufactured ICRBs. The power of these ICRBs was measured indirectly by charging a capacitor and measuring the voltage in the capacitor at regular and timed intervals. The power (P) needed to alter the voltage (and hence energy stored) in the capacitor over the time interval between readings was then calculated through where C is the capacitance, V 2 is the measured voltage, previously measured voltage and dt is the time interval between readings). A bespoke measurement system was used to measure the voltage of these ultra-low power devices while charging a 1 aluminium electrolytic capacitor.
The investigated configurations of ICRBs are displayed in figure 2, where (a) used two GTLSs on a PV cell without reflective casing, (b) used two GTLSs on a PV cell with reflective casing, (c) used two GTLSs sandwiched between two PV cells surrounded by reflective casing, (d) used one GTLS surrounded by three PV cells and reflective casing, and (e) used two GTLSs surrounded by four PV The results of testing are shown in figure 3, which plots the power ge at which that power was produced. The PV cells used during this testing were of amorphous silicon type, chosen due to their superior performance at converting low intensity Operating principle of an indirect conversion radioisotopic battery.
Gaseous tritium light sources (GTLSs) consist of a glass ampule coated on the inner surface with a thin layer of phosphor and filled with pressurized tritium gas. GTLSs therefore comprise two of the due to their sturdiness and A preliminary investigation of the most effective configuration of ICRBs was undertaken using The layout of the PV cells and GTLSs relative to each the power generated by the manufactured ICRBs. The power of rectly by charging a capacitor and measuring the voltage in the needed to alter the voltage (and hence energy stored) in the capacitor over the time interval between readings was then calculated through is the measured voltage, V 1 is the . A bespoke measurement low power devices while charging a 1µF 50V (1) , where (a) used two GTLSs on a PV cell without reflective casing, (b) used two GTLSs on a PV cell with reflective casing, (c) used two flective casing, (d) used one GTLS surrounded by three PV cells and reflective casing, and (e) used two GTLSs surrounded by four PV which plots the power The PV cells used during this testing were of amorphous silicon type, chosen due to their superior performance at converting low intensity The highest power was generated by having two GTLSs sandwiched between two PV cells (configuration (b)) and this was therefore the design which was taken forward for long testing.

Buried, long-life testing
The ICRB was installed in a waterproof enclosure conforming to IP65 standard and was potted with a transparent polyurethance resin (as shown in figure operation of the device. The ICRB was then buried at a depth of 90cm in soil of 19% moisture content (at time of burial) and was secured to a water pipe with plastic ties and a jubilee clip, as shown in figure 5. The hole was then backfilled and the performance of the buried ICRB was monitored over a period of two months via a bespoke sensor node, transmitter and data storage system.  The highest power was generated by having two GTLSs sandwiched between two PV cells (configuration (b)) and this was therefore the design which was taken forward for long The ICRB was installed in a waterproof enclosure conforming to IP65 standard and was potted with a transparent polyurethance resin (as shown in figure 4) to prevent moisture intruding during the buried operation of the device. The ICRB was then buried at a depth of 90cm in soil of 19% moisture content (at time of burial) and was secured to a water pipe with plastic ties and a jubilee clip, as shown in The hole was then backfilled and the performance of the buried ICRB was monitored over a period of two months via a bespoke sensor node, transmitter and data storage system.
Indirect conversion radioisotopic battery in waterproof enclosure and potted with markings ready for burial (lid not shown).
Power against voltage curves for the entally tested configurations of ICRBs as measured by charging a capacitor.
The highest power was generated by having two GTLSs sandwiched between two PV cells (configuration (b)) and this was therefore the design which was taken forward for long-life buried The ICRB was installed in a waterproof enclosure conforming to IP65 standard and was potted with a to prevent moisture intruding during the buried operation of the device. The ICRB was then buried at a depth of 90cm in soil of 19% moisture content (at time of burial) and was secured to a water pipe with plastic ties and a jubilee clip, as shown in The hole was then backfilled and the performance of the buried ICRB was monitored over a period of two months via a bespoke sensor node, transmitter and data storage system.
Burial of indirect conversion radioisotopic battery showing the device secured to a water pipe at a 90cm depth. The complete set of measured voltage readings from the burie with the temperature local to the battery which was read by a temperature sensor attached to the outside of the battery enclosure.

Figure 6. Measured voltage of
After approximately 20 days the temperature sensor degraded, most likely due to water encroaching onto the measurement mechanism, and thereafter produced unreliable data the sharp spikes in temperature readings equipment. Meanwhile the ICRB voltage shows a decline over the from a starting reading of 1.8115 reduced to half of its starting value. life of 12.3 years. Therefore, the life of the radioisotope. This is primarily due to the creation of impurities and the ejection of phosphor a A further phenomenon revealed du output of the ICRB and the local temperature. This relationship is shown more clearly in figure 7, where it can be seen that each drop in temperature correlates to a rise in generated voltage there is a general rise in temperature reported up to 20 output of the ICRB beyond the decline expected by battery aging. The effect of temperature on the generation of voltage by PV cells was experi similar results. For this reason it is believed that this phenomena is primarily due to the temperature dependent operation of the PV cells in the battery.

Figure 7.
A comparison between the mea omplete set of measured voltage readings from the buried ICRB is presented in figure 6 with the temperature local to the battery which was read by a temperature sensor attached to the Measured voltage of ICRB and local temperature reading over a period of two months.
After approximately 20 days the temperature sensor degraded, most likely due to water encroaching onto the measurement mechanism, and thereafter produced unreliable data sharp spikes in temperature readings. The gap in readings is a result of a power loss in the logging Meanwhile the ICRB voltage shows a decline over the period of measurement of 1.8115V. At this rate of change it would take 7.2 years reduced to half of its starting value. The radioisotope used in this ICRB was tritium, which has a half life of 12.3 years. Therefore, the voltage of the ICRB declines at a rate 1.7 times faster than the half This is primarily due to slow radiation damage of the phosphor layer the creation of impurities and the ejection of phosphor atoms from the phosphor lattice [8 revealed during this testing was the close relationship between the voltage output of the ICRB and the local temperature. This relationship is shown more clearly in figure 7, where it can be seen that each drop in temperature correlates to a rise in generated voltage there is a general rise in temperature reported up to 20 days, which has also decreased the voltage output of the ICRB beyond the decline expected by battery aging. The effect of temperature on the generation of voltage by PV cells was experimentally investigated by Shaari et al. [9 similar results. For this reason it is believed that this phenomena is primarily due to the temperature dependent operation of the PV cells in the battery.
A comparison between the measured voltage of the buried ICRB and its local temperature. d ICRB is presented in figure 6 along with the temperature local to the battery which was read by a temperature sensor attached to the ICRB and local temperature reading over a period of two months.
After approximately 20 days the temperature sensor degraded, most likely due to water encroaching onto the measurement mechanism, and thereafter produced unreliable data as evidence by The gap in readings is a result of a power loss in the logging period of measurement of 24.4mV years for the voltage to be radioisotope used in this ICRB was tritium, which has a halftimes faster than the halfslow radiation damage of the phosphor layer through oms from the phosphor lattice [8]. ring this testing was the close relationship between the voltage output of the ICRB and the local temperature. This relationship is shown more clearly in figure 7, where it can be seen that each drop in temperature correlates to a rise in generated voltage. In addition has also decreased the voltage output of the ICRB beyond the decline expected by battery aging. The effect of temperature on the et al. [9], which revealed similar results. For this reason it is believed that this phenomena is primarily due to the temperature sured voltage of the buried ICRB and its local temperature.

Conclusions
Indirect conversion radioisotopic batteries were experimentally investigated as to their optimum configuration which was found to be a sandwich of GTLSs between two PV cells. An ICRB was successfully buried and tested over a two month period proving the durability of this class of battery to this type of environment. Buried long-life testing of the ICRB found that the device would halve in voltage output every 7.2 years. This decline in voltage was due to the decrease in activity of the radioisotope (its half-life) and slow radiation damage of the phosphor layer. The voltage generated by the ICRB is affected by the temperature at which it was operating, which is due to the temperature dependent performance of the PV cells.