Sourcing Hydrogen Equipment Part 4— Solar Panels and Inverter / Battery Backup
The front end, the “The leaves” of our hydrogen fuel tree, our interface to the sun.
In this, part 4 of the series, continuing from part 3 below, covering everything needed for a practical large household or small community example off-grid solar hydrogen system, I will cover the panels and inverter selection.
The energy captured by this part from the sun, is what will power the hydrogen generated by our system.
The electrolyser detailed in part one is what this component needs to consistently supply, in a way that the average power is set by what the electrolyser needs to consume.
In addition, it has to supply all the hydrogen “losses” in the system, since those exist as long as we are processing hydrogen, ultimately they all have to come from the solar panels complement.
The “losses” in the hydrogen production processing chain from generation to consumption in the system are summarised as follows:
- Electrolyser loss — this is the energy lost in the electrolyser to heat. It is the remainder of the ratio of electrical energy into the device vs the energy in the output hydrogen. The enapter electrolyser efficiency is easily computed using the 2.4KW electrical energy rating vs its stated 1.0785 kg/24hrs H2 production rate, knowing 33.33KWh/kg energy content in hydrogen, we can take 33.33*1.0785/24 as the energy mass flow rate of the hydrogen from the electrolyser, giving us 1.48KW energy in the hydrogen output, so the efficiency is 1.48/2.4 = 62.4%. The heat energy out then is 37.6 % of 2.4 KW is approx 900W; about the power of a single economic room heater.
- Drier loss — this is the electrical power required to drive the hydrogen drier, immediately acting on the hydrogen generated from the electrolyser. The electrical rating of the heater gives us this loss directly from the specification, it is stated as 300W.
- Compression pump work done — this is the energy required to mechanically compress the hydrogen to 700 bar in the storage tank. The heat generated in this process is energy lost from the hydrogen compression process, whereas the isothermal work of compression results in mechanical compression energy being added to the hydrogen. In theory at least some of the isothermal compression energy is recoverable from the hydrogen at point of consumption. Theoretical Isothermal compression to 700 bar for H2 is 1.36 kWh/kg, whilst practical on-site values, measured in vehicle refuelling applications is around 1.7 kWh/kg. In the case of our domestic application, noting also the internal operation of our H2 pump is effectively multi-stage (the pump does not pump entirely the full pressure range from 20 to 700 bar in each compression stroke, it is actually done in many tiny steps within the pump, due to clever valving arrangement, the throughput rate of around a kg of hydrogen per 24hrs, is only a tiny fraction of the typical rates handled in vehicle refuelling applications, therefore the heating seen in the pump would be minimal. So we should assume the 1.7 kWh/kg best case seen in vehicle refueling applications, would actually be our very worst case. Hence this is the figure used in our modeling analysis. The maximum heat output from the H2 pump then should be less than 1.7–1.36 ~ about 340 Whrs per kg (since the isothermal component, 1.36 kWh/kg, is being added to the hydrogen as something other than heat), which at the rate we are processing /24, so maybe 12 to 15 Watts heat continuously. Notice also this heat is dissipated by the action of the exhaust air from the air compression piston, which would be cold air, due to being rapidly decompressed from the piston output, this is passed along the jacket of the H2 compression piston, cooling the latter, on route to the air being vented to room. The load supplied by the air compressor then is 1.7kWh / 24 ~ about 70W. We could obviously heavily downsize the air compressor identified in part 3, saving on installation cost there.
- Valve, piping, and regulator losses — this is the energy lost when the hydrogen is pushed through changes of resistance along its path through the system — these manifest as actually compression energy losses, as do all gas thermal losses in the system. In most systems these will not exceed maybe 10%. 90% efficiency is an often used rule of thumb, for system efficiency of valves and piping.
- H2 Fuel Cell efficiency loss — this is the energy lost in the H2 driven electrical generator. This is the ratio of electrical energy generated, vs the energy content in the input hydrogen. What is not converted to electricity has to be converted to heat. Our generator has an electrical supply rating of 10KW, supplied at 40% efficiency worst case, which means we could see 6KW of heat in that condition. In other words, at peak load it could consume a 16KW flow of hydrogen. To deal effectively with this, we have to consider practicalities of domestic loads. Some useful references for this exist in the case of UK, we have government studies known as “EFUS” (Energy follow up survey). These give sampled measurements of actual consumption, showing differences between households which are gas and electrically heated. In general, we see a 60% reduction in the electricity demanded, in households which are non electrically heated. In other words, 60% of a UK household’s energy needs are in heating. The most effective way of minimising the panels requirement of our solar installation then is to use the liquid cooling output from the electrical generator to perform the heating function in the domestic energy load. Isn’t it interesting that the power split of the domestic load matches more or less exactly the electrical / heat split output from the generator?
Totalling up the loads and losses to be supplied, that we see here, and adding to the expected domestic load for a non-electrically heated household, with a safety margin, gives us the total required panels complement. Although we still can’t know all this with much real accuracy until the system is fully modeled, this is a good practical rule of thumb, an initial point, from which the design model will proceed:
- Electrolyser Load: 2.4 KW
- Drier load: 300W
- Air Compressor load: 140W (Double the 70W found above, to give significant safety margin)
- EFUS Energy Affordability survey (Table 3.1) 75% percentile expected electricity energy consumption per year by a mixed energy household (2010 worst case) 5300 kWh / (24*365) = 0.6 KW
- + The energy lost in the generator supplying all of the above (0.6 times the loads above); 0.6*(2.4+0.3+0.14+0.6) = 3.44 KW
This implies that the total electrical load that needs to be supplied from the panels complement is 3.44+0.6+0.14+0.3+2.4 = 6.88 KW.
This is the total continuous load that the panels complement needs to supply, if we use the strategy of continuously generating hydrogen whilst consuming it. This would be the goal we need to meet, knowing the energy capacity of the panels, and the average daily hours of sunlight, in order to be sure we are capturing enough sunlight to keep the hydrogen charging up, over a period of time.
The daily hours of sunlight for UK is 4.9, so we should expect 3.9 kWh average energy per day from each 800W panel.
At 6.88 KW, our expected energy load per day is 6.88*24 = 165.12 kWh
The total number of panels needed to deliver this 165.12/3.9 = 42.33.
That looks like a scary number of panels, more than double the 18 originally estimated.
But this result is not conclusive, it is a worst case analysis, needed as an initial value for the design model.
This just emphasises all the more, why we need to use models to represent this system.
The secret to getting the input power requirement down, is in managing the power to the fuel cell mains supply.
That is the part that throws out 6KW of heat if we maxed it out to the full 10KW of its ability. This power dwarfs the power lost in the inefficencies of creating the hydrogen.
Intuitively, the other losses and energy costs in the system are manageable, within the original power budget, and it makes perfect sense to supply the domestic community heating needs, 60% of all the energy needed for the community, from the heat output of the H2 generator.
Of course there is a way of managing the entire chain to tailor the heat energy output by the hydrogen mains power microgrid power supply, to only what the domestic community needs, thus minimising the required panels capacity, with battery backed inverter.
So we will need to monitor the heat output of the mains power supply, and compare this with what the community is demanding, it order to control it to just meet that demand, and no more.
We can do that by supplying more of the power consumed by the community microgrid from the battery inverter.
Recall the goal of the system is to produce more hydrogen than we consume, as well as backing up our system, so as to be able to provide hydrogen to others requiring it, the more of this we can do, the better.
So cutting down what we use of it via the fuel call based mains backup supply is good in any case, but might require more inverter backup capacity, since we are shifting more of the backup requirement and work done, over to the inverter.
We can’t really get much details of how to do that withoug modeling, or by trial and error with actual hardware, which is expensive.
Modelling removes the requirement to do physical trial and error, by enabling us to do it virtually.
So readers should note that the model and trialling of it, which comes next, will reveal more about the system which might affect panels complement, and inverter specification.
That said, it is necessary for me to identify here the solar inverter and panels.
I said I would, but note that there is a high degree of uncertainty in the selection of this, until after it has been modeled with all system components fully specfied, and all control algorithms doing what they need to. It all hinges on the model now.
The following system from Alibaba looks to be capable of doing everythign we need. If one is not enough, multiples of these, they are fully scaleable:
I won’t go too far into the specs of these for now, will leave that for part 5, which will include full details of the model. Needless to say I’ve been building it up in the background as we go along, this is a sneak preview what it looks like now:
I hope we will be done by Xmas, less than 20 days to go now, not long.
I appreciate Engineering can look messy, when we publish all the gory details of how we get to a particular design. But it is most important for readers to understand that what we are doing here, is undoing a very large pile of misinformation about hydrogen, to get the truth of how to use it routinely to replace fossil fuels.
It can only be done by domestic and community solar hydrogen. The sun itself dictates how we need to use it — distributed, this is the only way to get the power from it that we need to undo the damage done by living entirely from the energy of the planet until now.
