Power rationing in a offgrid solar PV system and in Apollo 13

Power rationing in a offgrid solar PV system and in Apollo 13

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Tags: solar

A blog post on how I ratioed power usage in an offgrid solar PV system and a musing on general power usage.

When we turn on the power at our homes, we don't give much heed to the power consumed. We expect the utility to give us as much power as we want. We have the money (assume you're middle class or greater). However, when you're constrained by available power as well as money, you think of ways to reduce or optimize its usage. I faced that 8 years ago, when I made a 100 W rooftop solar PV system.

A 100 W rooftop solar PV system with a battery bank generates and stores about 300-400 Wh of energy per day. That power is quite limited. Suppose you have to power a laptop at night, how long do you think it can run off this system? It depends. When you're running off the main power supply you don't think about it. Either the power is there or it isn't. But here the answer is more complex. If your laptop is doing lot of processing, like gaming, it probably will consume a lot of power - about 40 W. But if you are only browsing with a laptop, it consumes about 15 W of power. This makes a huge difference in how long you can your lapotp.

Back in 2012, I had a HP Pavillion laptop that consumed about 40 W maximum power. I had a 100 W solar PV system with a 12 V, 35 Ah battery and a 400 VA inverter. I connected an ammeter to the battery to see how much current and power is being drawn from the battery. I was playing an city building game called Pharoah. The inverter consumed 1 A for its operation and the laptop took another 3 A - this totalled upto 4 A of current and 48 W (4 A × 12 V) of power. At this rate, the battery storing 420 Wh would last about 10 hours. Then I wondered - Pharoah is a pretty old game relased in 1999. My laptop, bought in 2006 was more powerful than the desktop PCs in 1999. So, why does the laptop need so much power? I throttled the CPU deliberatly using an software (don't remember which). The software can force the CPU into idle states for certain % of the time. I throttled the CPU to see how far I could go without impairing the gaming experience. I was able to throttle the CPU to about 60% idle time and the game still worked nicely. The current drawn by the laptop reduced from 3 A to about 1.5A. The overall current drawn from the battery reduced from 4 A to 2.5 A, reducing he power consumption by about 40%. This translates to about 60% more backup time. Till this day, I continue to wonder why old games take so much of CPU processing power, when they can actually do with lesser.

When I told about this experience to my brother, he suggested me to watch the movie Apollo 13. Apollo 13 was the third mission launched to land people on the moon. However, en route to the moon, an accident occurred in the service module of the spacecraft, resulting in loss of power. The fuel cells required for landing on the moon was damaged, resulting in the aborting the moon mission. The command module of the spacecraft had some charged batteries and fuel cells - these had to be conserved for Earth reentry. The ground crew on earth had to develop methods to operate the command module under low power conditions. The crew had to stay in the Lunar Module of the spacecraft. The Lunar module, used for landing on the moon too had limited power as the power reserves were not designed for extended use. The crew had to stay in freezing temparatures of 3° C. Also they had to ration water, which became limited due to the loss of fuel cells in the Service Module that produced water. However, with lot of work and some improvisations, they managed to bring back the three astronauts home.

Global warming threatens our modern civilization. We on earth are used to living on comfortable life with no heed to power consumption that causes global warming. The newer 'smartphones' consume lot of power running software and deploying services that becomes bloated everyday. One of the reasons why my blog uses simple html instead of a big showy wordpress site is that its quite light. You don't need a fast internet connection or the latest phone/laptop to load it. It won't run heavy javascript ads on the background that taxes you by eating your CPU's processing power or memory. We need to reduce our power usage. Like the crew who managed to bring back the astronauts safely under severe power restrictions, we need to develop ways to reduce power usage and global warming.

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