Thermophotovoltaic waste to energy backup generators can close the case for electrification
Summary:
With thermophotovoltaic cells now achieving 44% efficiency at 1435℃ efficient low-cost backup generators are possible that can consume residential waste and biomass directly. This enables colocated ~$0.03/kWh on demand power generation sufficient to complement ~0.03/kWh solar and batteries for full grid independence. Average grid costs of ~$0.08/kWh can be avoided, dramatically reducing electricity costs and greatly accelerating electrification. Development and commercialization of thermophotovoltaic waste to energy backup generators is quickly needed.
The opportunity:
As described separately, solar cladding for buildings and battery systems can achieve ~$0.03/kWh costs directly to the consumer and scale to meet ~90% of our total energy needs. Solar is now the lowest cost source of energy and this does not just apply to grid scale solar farms. Without subsidy, rooftop solar in Australia is now as low as $0.03/kWh USD. Large battery systems in China now cost as little as $65/kWh which corresponds to around $0.02/kWh per charge cycle. Solar and battery costs will continue to come down. The effective average cost of a residential grid connection is around $1,000 per year. Some will pay more, and will be more incentivized to defect from the grid sooner, while others will pay less. With grid defection, relatively fixed grid costs have to be covered by fewer customers resulting in higher costs per customer which leads to yet more grid defection - a vicious circle.
Intermittency is still a problem even with large solar and battery systems that are designed around the winter generation case, although this is more of a problem at higher latitudes. This is preventing the even faster adoption of this technology. A low-cost backup generator can mitigate this problem, but in order to achieve this a much lower cost fuel is ideally needed. Power from backup generators might typically cost on the order of $0.30/kWh due to the high cost of fossil fuels. Diesel at ~$3.50/gallon costs around $0.10/kWh of thermal energy which at 40% efficiency is around $0.25/kWh of electrical energy. Backup generators are becoming a sunk cost for many households in the U.S. due to grid outages associated with storms and wildfire risk. Increasing the utilization rate of backup generators by using them to mitigate solar and battery intermittency and to obviate the need for alternate large generating capacity is very appealing.
Bulk biomass like forestry slash and sawmill waste might cost around ~$0.01/kWh of thermal energy, although refined biomass like wood pellets costs significantly more. Municipal waste might cost negative ~$0.01/kWh of thermal energy - it costs money to take it away. The average household produces around 3 metric tons of waste per year. This includes organic matter. If converted to electricity at around 33% efficiency this equates to nearly 5,000kWh of electrical energy per year, or almost half of the average electricity consumption of a household. This is more than sufficient to mitigate the intermittency of high capacity residential solar and battery systems and can even cover electric vehicle charging. Using waste to energy generators at the household level greatly reduces the need for garbage pickup and landfills and can enable ~$0.03/kWh on demand electricity generation. The availability of ~$0.03/kWh year round electricity at the household level would greatly improve living standards, especially for lower income households, and greatly accelerate electrification. It would also greatly improve resilience to natural disasters. At ~$0.03/kWh it might cost around $3 to fill a car up with electricity, which is considerably less than the cost of filling up a car with gasoline or diesel. Thermophotovoltaic generators are a critical enabling technology that can greatly accelerate electrification and have a huge impact on improving economic prosperity.
Thermal photovoltaic generators:
Recent advances in thermophotovoltaics enable the construction of long-lived low-cost solid state backup generators with greater than 30% overall efficiency. This efficiency is likely to continue to improve. This enables direct waste to energy conversion in a highly distributed manner. Unlike steam turbines which need to be on the order of 50MW or larger to be efficient, the efficiency of thermophotovoltaics is largely independent of size, allowing for small efficient generator units. At around 1,500℃ the radiative energy intensity is almost 1,000x that of sunlight on Earth, and a very small area of solar cells is needed to generate very high power outputs. This suggests that very low cost thermophotovoltaic cells, perhaps even less than $10/kW, are achievable even with higher cost indium gallium arsenide phosphide (InGaAsP) cells. Balance of system costs will dominate.
A robust, efficient, and non-polluting thermophotovoltaic waste to energy backup generator favors the integration of a number of technologies including:
A high temperature combustion chamber with a thermophotovoltaic generator which is likely water cooled. The water cooling system can additionally be used for space and water heating. InGaAsP cells have lower temperature coefficients than silicon - they can operate efficiently at higher temperatures than silicon solar cells.
An automated feeding system perhaps including a chipper/grinder. This will depend on the feedstock and the size of the generator unit.
A recuperator heat exchanger that uses exhaust gases to preheat incoming air which is tolerant to clogging. Recuperator efficiency might be as high as 85% and this is the next largest source of inefficiency after the thermophotovoltaic cells.
A pollution control system that perhaps includes water scrubbing and an activated carbon filter. Water scrubbing can be combined with waste heat recovery for space and water heating. It might also be used to dry incoming feedstock. A venturi water scrubber can pump the exhaust gasses enabling air flow control and negative pressure operation which discourages leaks to the external environment. The extent and cost of the pollution control system will depend on the toxicity of the feedstock. In some scenarios waste materials, like packaging for example, might be designed for high energy content and low toxicity.
Requirements will vary and not all of these technologies necessarily need to be included, but total system costs are likely on the order of $1,000/kW. A residential application might only need a ~3kW generator system but commercial and industrial systems might be much larger. These costs are roughly in line with log burners, furnaces, generators, and other combustion based appliances, and a thermophotovoltaic generator can in many cases directly replace these products, mitigating additional capital costs. For example, integrating a thermophotovoltaic generator into a log burner with a wet back hot water heater cooling the thermophotovoltaic cells might only slightly increase total system upfront capital costs. Portable thermophotovoltaic generators that utilize waste or biomass and replace portable fossil fuel powered generators might also be developed. This might allow a single thermophotovoltaic generator to serve multiple users, sharing and reducing costs. Further along these lines a thermophotovoltaic generator could be integrated into a hybrid electric vehicle, allowing fast low-cost refueling and also serving as a large portable generator unit that could recharge households. There are many possible commercial applications for this technology and a large ecosystem of products could be developed.
They can scale:
Thermophotovoltaic generators can scale to meet the on demand energy needs of larger users such as industrial plants, large commercial buildings, and data centers. Global photosynthesis averages around 130TW with a little over half of that occurring on land. In comparison, global energy use is around 20TW. Assuming colocated solar and batteries supply the majority of our energy needs then there is sufficient global waste biomass production to serve the rest. Like solar, this is a technology that can scale rapidly with impressive cost reduction curves and operate in a distributed manner that bypasses the time and cost of grid connections. This enables cost effective colocated generation for situations where solar availability might be limited. For example, high population density cities, industrial plants, and data centers where energy demands may exceed colocated solar capacity. With sensible management, increased markets for biomass can also increase the demand for reforestation and the removal of deadwood and undergrowth that might otherwise cause wildfires.
Integrated battery storage:
Battery storage might be integrated directly into a generator unit. This can greatly reduce the cost of battery installation as it largely avoids the need for electricians. There is typically an almost 10x cost difference between a home battery storage system and the battery in an electric car. This is largely due to installation costs and the need to integrate with the grid. For example, a 13.5kWh Tesla powerwall costs around $15,000 installed, or around $1,100/kWh. By integrating the battery system with a separate generator system installed costs comparable to electric car batteries can be achieved. The generator with integrated battery storage can then simply plug into a house via a generator inlet box or a transfer switch. This can greatly increase flexibility and reduce installation costs. A hybrid electric vehicle with an integrated thermophotovoltaic generator might also serve this function.
Much lower cost off-grid energy:
The average cost of all grid supplied electricity in the U.S. is currently around $0.013/kWh while residential only is around 0.16/kWh. The development of thermophotovoltaic generators would critically enable colocated solar and battery systems and full grid defection with a dramatic reduction of average electricity costs to around $0.03/kWh. Energy is at the center of the economy and these cost benefits would permeate through to almost all sectors and have a huge positive impact on prosperity and standard of living. This would also avoid the need for expensive grid upgrades, would allow for the removal of transmission lines, including those that can cause deadly wildfires, and would greatly increase resilience to natural disasters. This critical enabling technology makes a much lower cost, cleaner, and higher performance energy future possible.
Status:
This technology likely does not require extreme engineering to develop and we do not have any intellectual property around it, although it would be fun and the water scrubbing and heat recovery approach is interesting. We are not currently actively pursuing the development of this technology. However, it would make the world a much better and more resilient place and we wish to strongly encourage others to develop and commercialize it.