Project Details

THE CHALLENGE

Our challenge is to design an energy storage system that will power a surface lander or rover on the surface of Venus for at least 60 days so that there is a viable energy storage capability for long-duration exploration missions.

OUR SOLUTION

WLPT: Wireless Laser Power Transmission

Project Details

Wireless Laser Power Transmission means transmitting energy through a laser beam in various media. In this project, a transmitter located on an aerial vehicle's power generation side would transfer the harvested power to a receiver located on lander energy storage through the CO2(carbon dioxide) atmosphere of a Venus mission. This would require transmitting power at radio frequency (RF) due to the presence of extensive cloud cover that would block other frequencies. The aerial vehicle would descend to the lower altitudes of the atmosphere to transmit its stored energy from its onboard high-temperature batteries to high-temperature batteries on the lander. On both the vehicles, either high-temperature molten salt or solid electrolyte batteries, or even a solid oxide regenerative fuel cell system, would serve as the energy storage medium. A rectifying antenna or “rectenna” constructed from suitable high-temperature materials would convert the beamed energy to direct current electrical power on the lander. Having the energy transferred to the lander, the aerial vehicle would ascend to the High Solar-flux upper reaches to recharge its batteries. Once these batteries were fully charged, the entire sequence would repeat. The lander would continue to perform as long as its components and systems survived, without being power-limited. The aerial vehicle could continue the secondary science investigations after the end of the landed element of the mission. The aerial vehicle can also serve as a communication relay between the Earth and the Venus lander. It would also support the transmission of data. This approach will also make uses of HOTTech photovoltaics which are feasible down to adequate solar flux (20 km) and high-temperature rechargeable batteries (which can be used in the upper reaches and on the surface) in a new way by separating the power generation and energy storage functions.

This approach has the potential solution to the Venus surface energy storage limitations and is feasible for powering a 60-day lander, which is a significant leap from the longest-lived previous landers.

Limitation of the priors

A burning challenge in supporting a lander for any mission duration is the lack of feasible options for power generation. All of the priors have limitations that make it quite impossible to operate and survive for a long-life lander. Let us see some prior potential options for power generation energy storage systems.

high-temperature primary batteries

Previous missions (i.e. the NASA Pioneer Probes) relied on primary batteries only, which survived for approximately two hours. some missions feature Li/SO2 non-rechargeable primary batteries as energy storage, which cannot adapt to Venus surface temperatures and are limited to the fixed capacity in the battery cells. It limits both the lifetime of a lander as well as the science payload. Even though there are possible technical solutions for high-temperature rechargeable batteries, we are gonna use it in our project partially.

solar arrays

The solar spectrum in the Venus atmosphere is different from that on Earth and varies significantly with altitude. Direct solar radiation can be neglected at altitudes below 60 km and upward solar flux is significant. The efficiency of the solar cell depends on temperature and impinging solar flux. Despite the lack of significant insolation on the surface, some observations point to the usefulness of high-temperature solar array tech. at higher altitudes. We are gonna also use this partially.

radioisotope power systems (RPS)

In Radioisotope power systems (RPS), or in particular radioisotope thermoelectric generators (RTGs), a radioisotope heat source (plutonium-238) is used with power generating thermocouples featuring thermoelectric materials designed for extended high-temperature operation. The RTG system was designed to operate in a very specific temperature regime. The hot side of the power-generating thermocouples operates in the 550- 1000⁰C range (depending on the specific thermocouple material used), with the cold side rejecting heat to either the vacuum of space or the specific planetary atmosphere where it is operating. All aspects of the RTG design (structure, thermal materials, thermocouples, etc.) are optimized for a specific environment. Although operation in a high temperature and high-pressure carbon dioxide environment such as Venus is technically possible, it would require a complete redesign of the RTG system to address operation in this challenging environment. The environment would pose a temperature differential unlike any previously experienced by an RTG system. The use of a dynamic converter technology (i.e., Stirling, Brayton engines) is also possible. However, the cost associated with such an extensive re-design and qualification of the heat source and converter technologies to serve a niche destination and environment such as Venus would be prohibitive and would be accompanied by a loss of the multi-mission benefit of the current RTG technology.

wind power

Some prior concept features the proposed harvesting of wind energy for power generation, at the surface of Venus. However, the velocity of these winds is very uncertain and likely to be too low (~2 m/s) or too sporadic to support a long-life surface lander architecture. There is a finite probability of wind speeds approaching 0 m/s. It is unlikely that a mission could be built around such a scenario. Another concern is the effect of wind turbines on seismic monitoring, which is a key goal of a long-life landed mission.

chemical heat sources

There is also current work on “chemical” heat sources involving the reaction of highly exothermic fuel/oxidizer combinations, which can be used in place of radioisotope heat sources based on plutonium-238, to power either an RTG or a dynamic RPS. Although these systems show promise for replacing the plutonium-238 used, they are essentially a primary power source limited by the mass of the carried fuel and/or oxidizer.

*Similar approaches involving the reaction of an electro-active species (i.e.lithium) with atmospheric carbon dioxide to form a type of “gas battery” would suffer from the same limitations.

Why WLPT

is practical?

WLPT: Wireless Laser Power Transmission method which takes a source of energy that is one location abundant and transmits this energy through a medium to a region where it is less abundant. The situation with Venus exploration represents an ideal circumstance for the implementation of the WLPT. Solar energy is abundant in the upper reaches of the atmosphere (even greater than on or above Earth) and is highly restricted on the surface due to the extensive and persistent cloud cover. An ideal Venus-based mission would involve transferring power from an aerial vehicle down to the surface. With Venus located at 0.72 AU from the sun, there is approximately double the solar intensity of our planet. This could support the collection and transfer of high levels of power if emerging high-temperature solar cells were used. Unfortunately, there are several reasons they are not feasible from such a perspective. Firstly, the challenge of the very slow rotation of Venus (6.52 km/h), resulting in a single Venus day that is equivalent to ~243 Earth days. This would place the Venus Synchronous Orbit at a distance of 925,000 km from the surface of the planet, resulting in significant power spreading when the transmitted energy would reach a lander. Secondly, due to the dense carbon dioxide atmosphere, atmospheric attenuation of a radio frequency (RF) signal would be too great (100’s of dB) to be practical. Therefore, the only concept featuring WLPT that is viable must feature the collection of the abundant energy above the clouds, descent to a lower altitude (taking advantage of the transport offered by the thick atmosphere), and transfer of the energy to the lander. Over the years of Venus surface lander mission studies, concepts have typically fallen into one of three categories in the extreme Venus environment:

1) landers utilizing active cooling and a pressure vessel to shield the sensitive spacecraft components and systems,

2) landers that are inherently robust under these conditions (e.g., high temperature, pressure-resistant electronics and systems), or

3) a hybrid approach that combines both of these concepts.

They would require batteries and other energy storage systems larger than the whole mass of the other things, which clearly shows the usefulness of our WLPT.

WLPT:

The Whole Process

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