Design the Solar Rack and the Electronics
The idea of a solar container isn’t new–in fact there are commercial versions available with some very interesting features–if you have a few hundred thousand bucks to spare. The notion is simple: take a container, probably a 40 foot standard volume one, add racks that can hold a substantial amount of solar panels. Install all the equipment required to convert the DC power from the PV panels to whatever flavor of AC you require. Plunk it down in an unshaded location close to whatever you need to supply power to. If you do everything right you can have reliable, un-interruptable power for virtually no continuing cost with minimal maintenance for many years. This approach can be used for both on- and off-grid systems, for residential or commercial use, as long as there is a space for the container. The equipment inside the container actually takes only a small amount of the total space, leaving the rest available for other uses. While zoning and aesthetic considerations limit the application, it’s a practical, pragmatic, portable solution in many cases.
The system we are building as an example is three-phase 120/208VAC with substantial battery backup for light industrial use, but the exact same layout and equipment can be used for single split phase 240/120 VAC in residential and commercial applications. The only difference is that 3 phase power requires at least two 3-phase capable inverters wired a bit differently and configured in software to supply three supply lines 120 degrees of phase apart at a voltage of 208 VAC in reference to each leg and 120VAC referenced to neutral. The exact same inverter I’m using (EG4 18K PV) can also supply split-phase 120/240 VAC with one inverter, though up to ten inverters can be connected in parallel for greater output power and to handle more solar power input. We’ll do a deep dive into hybrid inverters in future videos.
Design Considerations:
- Container: The container to be used is a 40′ high-cube container which means the dimensions are 40′ long, 8′ wide and 9’6″ tall with two full height, half width latching doors at one end. The containers will be painted light grey on three sides and white on the roof and south-facing side to reflect sunlight. The equipment and battery area inside will be insulated and faced with painted cement backerboard for mounting electronics and behind the battery racks. The ceiling above the batteries will also be faced with backer board with the remainder of the insulated space being faced with standard wallboard. A wall with an access door will be added to reduce the amount of space to enable efficient heating and cooling but still allow room for expansion as necessary. The remainder of the container will remain un-insulated steel, suitable for storage. An inverter-based minisplit heat pumpwill be added to control the temperature in the equipment/battery room.
- Racks: The tilting roof rack will consist of three racks with pivots welded along the spine with the mating pivots welded to the edge of the container. The first two racks are ten feet long and carry 6 Aptos 370W panels each. The third rack is 16 feet long and holds 10 Aptos 370W panels. For simplicity the racks will be rigid, though I have done the design work for a rack consisting of two sub-racks that are 2 panels high each, capable of being locked together or pivoting independently which would allow the top panel to lay flat on top of the container and the lower panel to lay flat against the side. This would permit easy transportation, which is unnecessary for my current application. I may add linear actuators and an optimal angle seeking control system to manage the panel tilt later.
- PV Panel Output: The twelve Panels on the two 10-foot racks will be connected in a series string. The (open circuit) voltage of the panels is 41.4VDC yielding a maximum string voltage of 496.8VDC at normal operating temperature of 25C (77F). The rated voltage is 34.5 and rated current is 10.72 for a panel output of 369.8 and a string output of 4438 watts, or 10,000 watts for the 12 panel rack and 3700W for the 10 panel rack. The shop I’m connecting this project to has a huge solar potential but I don’t want to poke holes in a 15 year old roof. I have a lot of unshaded space for ground mounts, so I’ll be adding a variety of those.
- PV input: The equipment I’m installing has 3 MPPT solar controllers enabling three independent strings of PV panels. The first MPPT controller has 2 panel inputs in parallel and that controller is rated at 25A. Controllers 2 and 3 are rated at 15A input each. The controllers are rated at 230 to 500VDC input which accomodates my 12 panel series strings. The maximum allowable DC input is 600V which means even the open circuit voltage (Voc) of 41.4V for the panels I’m designing for is comfortably within maximum specification for a 12 panel series string even considering the higher voltage that is possible on cold sunny days (we’ll do a short video on calculating maximum Voc in for lowest historic temperature at your location).
- MPPT Controller, Inverter, and Batteries: I’m installing EG4 18K hybrid inverters. Hybrid means that the inverters can operate either off-grid as a independent power source or in a grid-tied, synchronized manner to continue to use grid-supplied power when necessary and backfeed power to the grid if a suitable net metering contract is offered (see Grid Connection below). This is a remarkably versatile and capable inverter/controller/charging system that suits a lot of use cases. It can accommodate high voltage DC coupled solar panels, AC coupling to accomodate legacy PV systems (with microinverters or string inverters. It can work with or without batteries, can accommodate single phase 120VAC, split phase 120/240VAC, or in combination with two or more units, three phase four-wire wye-connected 120/208VAC with a common neutral (the most common 3 phase commercial/light industrial 3 phase power). It includes battery communication via CAN bus and RS485, supports a backup generator with both power connections and remote start integration. And various battery configurations. The batteries I’m using are EG4 outdoor-rated wall mount 48v 280Ah 14.3kWh LiFePO4. The EG4 18K inverter is outdoor rated, which is a plus for longevity, since the electronics cooling system doesn’t blow dusty air through the case to cool electronics, but rather relies on a heat sink system that keeps the electronics separate from the cooling air.
- Grid Connection: I expect to have an interesting time explaining my system and negotiating for a net metering agreement with my local power company. I don’t really care very much how that works out, this project doesn’t really hinge on economic breakeven. I live in an area with abundant hydroelectric power and relatively low population. Electricity is cheap compared to most places in the USA. But the grid isn’t becoming more reliable, and the ability to charge our EVs and ensure we have power regardless of what might happen to the grid is the primary driver. That, plus I really like making complicated things.