A measurement system verifies that solar cells produce sufficient power to enjoy your morning cup, even when far from the power grid.
After a sabbatical of a few weeks, I’m happy to be back writing for the Test Café blog. And what better way to return than with a column that combines both test and café? Today I report on how much solar power is required to brew an espresso, and whether solar power is a feasible method at all for that task.
Here’s some background. A year ago, my wife and I acquired a small travel trailer that we use for off-the-grid camping. I reported earlier on my experiment of amplifying weak cellular signals to create a robust internet connection from rural Wyoming. Since that time, I’ve integrated a rotatable Yagi microwave antenna onto the roof of the trailer to capture and amplify seemingly useless cellular signals. Now I often enjoy being the sole camper in a remote campground to have reliable voice and internet connectivity. I’ve even uploaded a couple Test Café columns from the middle of nowhere using this scheme.
Now the coffee part. I like coffee, but I love espresso. The traditional way to make coffee while camping is to heat water using propane burners, and then brew the coffee in a French press or drip system. Battery power in a travel trailer is limited for relatively low current devices, and that surely doesn’t include 1000-W espresso machines.
Perhaps it doesn’t have to be this way. Having previously installed 200 W of solar panels on top of our camper, charging a 100 Ah lithium battery, we have plenty of power for camping. So much that, like California, we may have to pay neighboring campers to take some (Just kidding, but we have ample recharging capability). Could we just plug our Nespresso-brand espresso maker (Figure 1) into an outlet, and brew a nice foamy espresso on demand?
There are two key challenges. The first is that no coffee maker runs on the 12V natively available from the lithium battery. They run on standard household AC. This can be solved with an inverter placed close to the battery with very thick but short cables connecting them. The inverter would then power a standard 120V electrical outlet. That’s exactly what I did, placing 00 Gauge cables (also known as 2/0 AWG Gauge) between the inverter and the battery. At 78 µΩ per foot, the pair of three-foot cables adds just 468 micro-ohms of resistance. At 100 A, this equates to a voltage drop of 0.047V, essentially negligible from a 12V battery.
The second challenge is how quickly the battery will drain. A 1000-W appliance could easily pull 100 A out of the battery when inefficiencies are taken into account. Furthermore, coffee is typically consumed in the morning, when the battery is at its lowest charge level. After a night of power consumption without solar charging, it’s not unusual for me to find that just 60 Ah of battery capacity remains in the morning. A quick calculation shows that a 100 A drain would kill the battery in just 36 minutes. Yikes! I better drink all my coffee quickly.
But is this really the case? Does a 1000-W appliance continuously consume 1000 W? Or is it a rating of peak power? Or something in-between? How much power does it take to brew an espresso anyway? This is a job for test and measurement.
Fortunately, when I designed the solar and battery system, I installed a robust measurement system based on the Victron BMV 700 battery monitor. As Figure 2 shows, the battery monitor can measure voltage and current. It measures current through a precision 0.1 mΩ shunt resistor capable of supporting currents up to 500 A. The BMV 700 merely measures the voltage across the shunt. A critical feature is that the BMV 700 can perform the function of an integrating ammeter. By integrating the current in and out of the battery over time, and with knowledge of when the battery last reached a 100% state of charge (SOC), the battery monitor can accurately measure the net charge removed from the battery and determine the battery’s remaining charge level. This charge value is just what I needed to make the espresso measurements. A nice added feature of the BMV 700 is that it has a Bluetooth connection, so I can display all the measurements on my iPhone.
Figure 2 shows the simplified diagram of the solar/battery and measurement system. A solar charge controller uses pulse-width modulation to optimally charge the lithium-ion battery, stopping when it detects the battery is fully charged. An inverter generates 120 VAC to drive a load up to 1500 W. The measurement system measures the voltage across the shunt to measure current in and out of the battery. The battery voltage is also measured.
To make the espresso calculations, I made the measurements at night, so no current would be coming from the solar panels. I turned all appliances and lights off, so the inverter was the only load. I wore a headlamp while performing the measurements. I plugged the Nespresso machine into the inverter and turned it on. Immediately, it was pulling 110 A out of the battery. The battery voltage dropped from 13.5 to 12.6V due to its internal resistance. A quick calculation was that the Nespresso machine, rated at 900 W, was consuming 1386 W through the inverter. This went on for several seconds as the Nespresso machine warmed up, then the current dropped to under an amp.
So, how much power does it take to brew an espresso? 4.4 Wh.
Ten espresso shots consumed 3.5 Ah of capacity, or 0.35 Ah each. When the Nespresso machine is drawing a shot, 110 A are being drawn and the battery drops to 12.6V under the load. 12.6V × 0.35 Ah = 4.4 Wh of power per shot. Between shots, the power consumption drops to near zero.
At 0.35 Ah, the battery has capacity for making 285 shots, though 200 may be a more practical number in the morning before the battery is fully charged. But here’s something else. By mid-morning, the solar panels deliver well over 10 A of charging current. That’s 28 shots per hour without using any battery capacity at all—just from the morning sunlight. 28 shots/hour is coincidentally my normal coffee consumption rate. Not only does the system have plenty of capacity for espresso on demand, it has enough to support an entire coffee shop for a campground, complete with wireless internet. Perhaps I should name the coffee shop Test Café (Figure 3).
Should I have been surprised by the results? Not really. Here are the theoretical calculations based on the energy to heat water enough to brew espresso. These are double shots, close to two ounces each. Espresso has a large amount of foam, so I weighed a typical shot at 37 g. I estimated that the temperature of the water was being raised from 20º to 90º C and used 4.18 (the specific heat of water) as the specific heat of espresso. 37 g × 70º rise × 4.18 = 10,826 joules. A joule is a watt-second, so divide by 3600 to calculate the energy required, resulting in 3 Wh per espresso shot. This compares well with the 4.4 Wh measured. The 47% additional inefficiency is likely due to a combination of inverter inefficiency and the heating of the heating element and water vessel.
Now, excuse me as I brew another shot. The Café is open.
—Larry Desjardin is a regular contributor to EDN‘s Test Café. He served in several R&D and executive management positions with Hewlett-Packard and Agilent Technologies. Contact him at firstname.lastname@example.org.