Labora: Mobile energy accounting

Social aspects aside, energy consumption is critical when we work on the road, “in the middle of nowhere”, in the van. Some notes on energy accounting with solar panels and batteries.

Starter notes

• Energy always runs out faster than you think. There is a permanent energy shortage on board. It is important to understand where it is consumed and where it comes from. We really think of everything to minimize our energy footprint and still keep it livable.
• This is an initial review of the mobile office off grid energy system.
• Main conclusion: Off grid running an energy system with normal consumers is perfectly feasible with solar panels in particular.
  • Large consumers such as electric air conditioners are excluded in this sense.
• A set of mainstream solar panels (~800Wp) on the roof of a bus provides a sufficient amount of energy.
• Choosing energy consumers extremely critically and using them consciously creates an equilibrium between production and consumption.
• The following is a brief analysis…

Balance

As it stands, we can work a working week where the batteries have a little less capacity each day. At the weekend, they can then be fully filled again. That is a basic idea and it works. In practice, we are not put together like that and work through the weekend and sometimes not work during the week. On net, it comes down to the same thing.

The balance consists of a supply and drain of electricity, there is production and consumption…

Batteries

The entire battery pack is actually not that big. There are 6 kWh available and that is enough to form a buffer between supply and drain of electricity. That buffer gets a little empty and a little fuller, depending on many factors. To name a few: Is the car running? Is the weather nice? Are we going to cook extensively? The nicest thing is if the battery never runs out of power but does get close to, say, 20% as a reserve.

How big is the battery?

The role of the battery is to be a buffer. Some goes in and some goes out. During the day, you consume energy but also make energy. You make more than you consume during the day and at night it is exactly the other way around. So the capacity is at least the sum of the energy to cover the night, plus some extra if there is little sun during the day, plus an extra percentage to avoid a “black out”.

If you meet the condition that consumption “U” per day is about the same as production “P”, then choosing a battery of “P * 2” is sufficient. This is, of course, a rule of thumb.

If you stand in the sun for a long time and make regular trips, then less will suffice. If you are in one place for a long time and the weather is not particularly good then you may want a larger battery capacity. Of course, all this only works if production and consumption are reasonably matched over a long period of time.

Production

The LFP cells (LiFePO4) are charged by the sun (810 Wp), the motor (800 W) and possibly shore power (600 W). In doing so, you want to use as little shore power as possible and that is the practice at the same time. The energy comes from the sun and is generated while driving. How much is that?

It is realistic to say that, on average, you do drive an hour a day for shopping or to get to another place. With that, you do grab a little kWh on average. The total panel output turns out to be around 2 to 2.5 kWh on average. The average yield, without shore power, is therefore about 3 kWh per day. Sometimes a bit more and other times a bit less. So you should be able to buffer that with the battery.

The loading capacity is ~50% of the total capacity per day.

Consumption

What is the energy consumption per day? A rough indication:

• Major incidental consumers: These are in the kitchen, coffee, tea and cooking. Kitchen: ~600 Wh.
• Small incidental users: Lighting, pumps, etc.: ~150 Wh
• Continuous consumers: A large part is sneakily gobbled up by 24/7 appliances fridge, ventilation, chargers, 230V inverter, lighting and so on. Small continuous consumers: ~650 Wh.
• ICT consumers: Computers, all AMD Ryzen based, will also be large consumers when used intensively.
  • The integrated desktop server, together with the large monitor, is ultimately the biggest consumer. That is also where the big gains can be made – and they have been achieved. An interesting fact is that the desktop on Linux consumes only 16 W in idle mode, including switch and AP, and if a Windows virtual machine (VM) is started, the total is only 30 W. Much more than 5 to 15 W extra during intensive use is not added. The vmpower script also provides another hefty saving, just like an almost direct 19 Volt DC feed from LFP, omitting 230 Volt conversion. Including economical monitor, a consumption of less than 60 W is quite realistic. 8 hours of use… Workstation and server: ~500 Wh.
  • The ThinkPad is amazingly economical, by the way. For 10 hours, Laptop: ~300 Wh.
• There are always additional setbacks, 20% is the assumption, the offset.

That is less than production and all in all a happy ending!

Featured image courtesy: Solar Energy Technologies Program. U.S. Department of Energy.