Emergency Power in the Woods |
Background
Being able to power our shacks or mobile gear in emergency situations requires a power source that doesn't rely on the commercial grid. Some of us have generators, some have deep-cycle batteries, some of us have both. If you have batteries, the question is how to charge them. The easy way is a float charger which is good for keeping the batteries topped off while the commercial power grid is operating. Once the grid becomes inaccessible, the battery begins its discharge. If the batteries are connected to solar power, however, you're completely independent of the power grid. The ARRL has a chapter on solar power in its Emergency Power handbook as well as more detailed articles on batteries and inverters. A few weeks ago, I discovered that Barr Camp, the midway point on the Barr Trail between Manitou Springs and the summit, has a solar power system. After talking with Neal Taylor, one of the camp caretakers, I found out that the solar power system they have can store 50 kilowatt hours of electrical power--now that's serious. I asked for a tour of the system and here's my report. |
Theory
Before the details, let's take a look at the fundamental components behind a solar power system. The basic scheme consists of a handful of components, all of which are sized to the anticipated load. There are five elements shown in Figure 1, below. The solar panels typically produce 12vdc, but the maximum current which can be produced under full illumination varies with size of the panel. Panels can be connected in series to produce higher voltages, and in parallel for more current. If a long cable run is necessary from the solar panels to the rest of the equipment, panels in series producing 24 or 48 volts will be more efficient in terms of power transport. The solar panel output is fed to a charge controller which has two main jobs. First and foremost is to charge the batteries efficiently and maintain their state of charge without overcharging. Secondarily, the charge controller disconnects the solar panels when there's not enough energy to charge the batteries (e.g., at night) so that the solar array doesn't become a current sink. Next in line are the batteries. These are typically deep-cycle lead-acid batteries which can be discharged and recharged many times without significant degradation. A circuit breaker and main switch panel ensures the safety of the downstream system. The inverter, of course, converts the DC supplied by the batteries to 120 vac used to run appliances, power supplies, lights and other equipment. |
Figure 1 -- A general model of a solar power system.
A note on deep-cycle batteries Deep cycle-batteries are different from automobile batteries because the latter are typically kept at a high state of charge and are only used to supply a large amount of current for a few seconds. They're not meant to be completely discharged and then recharged. The lead plates in automobile batteries are thinner, and that's OK if the battery doesn't lose much of its charge. Deep-cycle batteries, by contrast, are made to be run down to almost nothing and then recharged. Their lead plates are much thicker and they can support the greater depletion required by significantly discharging the battery. |
Practice
My first understanding of the size of the power system at Barr Camp came from a conversation in which Neal told me that they needed to replace the twenty-four six-volt batteries (weighing 130 lbs each). Neal agreed to give me a tour of the system and this is what it looks like. The solar array is located on a south-facing hillside about 200 feet from the main cabin. It's composed of 12 120-watt solar panels (see Photo 1, below). |
The solar panels are connected four in series, three in parallel so under full illumination, the system produces 48 vdc at 30 amps. The array tracks the sun and is driven by two small 48v motors, one located in the collar on the support shown in Photo 2 which controls azimuth, and the other in a screwdriver configuration (in the arm at the left of Photo 2) controlling elevation. |
Photo 1 -- The solar array at Barr Camp |
Photo 2 -- The back side of the array |
The phototropic system uses a sensor which detects the sun-shadow boundary and keeps it in a constant location. The controller also contains logic which drives the array back to the east after a period of time in darkness. |
The cables from the solar array terminate at the charge controller shown in Photo 3. The charge controller, batteries, cutoff switch and inverter are allocated in a room of the main cabin. The charge controller is programmable and allows for different charging regimes (e.g., bulk charge, float charge) based on different state-of-charge set points. The front panel shows both the source voltage and the battery voltage. The controller is an Outback model MX60 which can handle up to 60 amps at 48v from the solar panels. |
Photo 3 -- the charge controller |
Photo 4 -- inside the battery coffin |
Where the system gets serious is in the battery coffin. Because these are lead-acid batteries, ventilation for the hydrogen outgassing is required. Temperature control is a good idea, too, because batteries lose efficiency at low temperatures and can be damaged at high temperatures. For this reason, special enclosures are generally built to house the batteries. in Photo 4 below, you can see about half the batteries in the coffin. The green container in the center contains distilled water used to maintain the electrolyte levels in the batteries. Neal checks all the cells twice a year and does another random check two more times each year. Low electrolyte levels will decrease the charge the battery can hold and can damage the lead plates. |
Barr Camp has 24 UL16 batteries (6vdc, 350 AH, ~130 lbs each). These are wired eight in a series for 48vdc, and of course there are three strings which are wired in parallel. The batteries can store 50 Kwh of energy. Between the batteries and the inverter is a manual cutoff switch and an overload circuit breaker. The inverter is a Trace 4048 which is a 4kw, 48v inverter. Given the storage capacity of the batteries, the inverter could run at full load for about 12 hours with no input from the solar panels. |
Photo 5 -- The inverter |
Putting it all together in one diagram, the power system at Barr Camp is described by the diagram below. |
Figure 2 -- Block diagram of the Barr Camp power system
Why?
The obvious question to me after learning all the numbers from Neal was "what was the load this was sized for?" Neal explained that their biggest load was a refrigerator. They also had a couple of lights, some battery chargers for their SAR radios, and a laptop computer. But the real reason for the power system… "Well, it's the toilets." It turns out that a number of years ago, the Forest Service insisted that Barr Camp no longer use the latrine-like arrangement. A fund-raising campaign was started in order to purchase the composting toilets--two for $20,000. Composting toilets require only a small fan to operate properly, plus a light bulb for for convenience. The fans in the Barr Camp toilets are small motors requiring a few watts at 24v. The light are 28w fluorescent tubes. After the toilets were purchased, the money left over bought the power system--about $25,000 worth. So there's no 4 kw load at Barr Camp. Or a 3 kw load, or a 2 kw load. It's about a 100-watt load...plus the fridge. They could operate for a couple of weeks in total darkness. Now that's serious emergency power. |
References: Bryce, Michael. Emergency
Power for Radio Communications.
http://amasci.com/tesla/wire1.txt |
Al Glock, KC0PRM