What’s the plan
I have a cabinet containing a few small servers, cable modem, router, switches etc. and the hub of a security DVR system. Externally, a couple of wifi APs and IP cameras running on a DIY POE. In total it consumes about 250 watts and was running off a APC Smart 1000VA UPS. The UPS was quite inefficient, always running hot and consuming over 40 watts even with no load connected.
I like the idea of a reasonable battery backup and also reducing the power bill with some energy coming from a small wind/solar system on the roof. More of an experimental project at this stage than a serious alternative energy system. So, the plan is to have everything operate from a nominal 24V DC battery backed supply with power from the grid, wind and solar. The wind generator has to be relatively small so as not to upset the neighbours or local council.
This is a work-in-progress.
Load equipment requirements
For this to work, all of the computer equipment, network gear, security cameras etc. must run from 12V DC or be happy with 20-30V DC.
The cable-modem, router, network switches are all happy with a clean 12V DC. The 2 smaller PCs running as servers are also happy with 12V DC and draw no more than 4 Amps each. The larger main server is currently running on a standard 230V grid supply and drawing about 80 watts average. So I will have to replace this with a power supply tolerant of 20-30V DC.
The POE connected gear is already fitted to operate with 20-35V AC/DC supplies. Considering cable volt-drop it is best to power the cable at close to 35V DC; say 32V DC. A small step-up regulator can handle this.
It’s mostly assembled from off-the-shelf power supplies and modules. I looked at mounting everything on a gear-tray and wiring it all up. But quickly decided that method would be too time consuming and would only produce a one-off.
So I designed a circuit board to combine regulator modules, diodes and connections.
Outputs on the left, battery-solar-wind are the large green terminals on the right, power diodes on the red heatsink and additional filtering on the outputs.
Heatsink and cooling
There are eight high-current schottky-barrier diodes on the red heatsink; should have been matt black but I only had red paint. The holes allow access to the regulator module voltage and current adjustments.
Depending on load, it will probably need a plastic cover and small fan just to keep the regulator modules and main heatsink from running warm. The cooler the better.
Grid power comes from two 24V 10A supplies fed from a PFC circuit to provide main power, with a separate smaller supply maintaining a float charge on the batteries when there is no wind/solar. When wind/solar is available, the voltage is slightly higher and delivers energy to the load reducing the grid load.
Due to the regulator modules, the 24V supply is safe to drift with supply and battery condition over 20-30 volts without affecting the equipment.
The mains supply voltage (24V) must be high enough not to pull the battery voltage too low but low enough that the alt-p takes over when available.
For maximum life, the batteries should be kept cool, below 20-degC and not floated above 27.6V. There are diode volt-drops to consider that will vary with load. The alt-p supply voltages must be limited to maybe 29V to prevent the batteries being overcharged. A bit of experimenting and monitoring will be required to set it up. If the alt-p provides enough power, a smarter charging system will be required.
Ideally, deep-cycle batteries would be used, but these are very expensive here in New Zealand and will not normally be cycled. So I’m using some 12V 18AH SLA batteries.
Standard PC power supply replacement
This circuit board can also be assembled to replace a standard PC power supply, allowing operation from 20-30V DC.
The four switch-mode regulators would be adjusted to deliver 3.3V, 5V, 12V-1 and 12V-2. The diodes and heatsink are not required.
The 32V and 12V outputs are voltage adjustable
Alternative power options
The original plans was to to have a small roof mounted wind generator and solar panels supplement the grid power. The tricky part is combining the various power sources and keeping it simple, reliable and low cost, without sacrificing too much efficiency.
As a first step, a simple arrangement of dual schottky barrier diodes will do. If this proves to be inefficient or difficult to manage, smarter solid state switching may be required.
Update – August 2016
The current server cabinet has been operating for almost a year now.. It consists of a mains powered PFC unit driving two 24V 10A power supplies feeding the above board. A 190 watt 24V solar panel contributes up to about 160 watts on the sunny days – more often than I originally thought. There is about 60 Ah or 24V battery backup.
Several computers (Linux servers) and all peripheral equipment now runs directly from 24V or 12V and the outdoor equipment runs on 32 VDC POE. Total system load is around 180 watts most of the time.
The cabinet is out of the way and not often accessed. Updates and support is done by remote access. The cabinet is ventilated and puts excess heat into the house – efficient in winter. A safety monitoring unit tracks temperature and smoke and can shut it all down. The batteries are located in a more accessible location where the temperature is more constant year-round.
The power supplies and distribution are on one end. Networking and CCTV on the other end. The tall computer is no longer used. Two mini-ITX boards and two ex-lease Lenovo M58 Thinkcenters (dual-core 3GHz) run servers. All modified to operate on 24 VDC.
The mini-ITX boards use about the same power as the Lenovo machines but are no where near as quick. These will eventually be swapped out.
There is also one ARM based Cubieboard with a small SSD running a mail server. More about this here.
Updates have always had low cost and low power consumption in mind.
I would like to swap out the Intel machines for ARM based servers to reduce power draw even further. The hardware is available and reasonably cheap already, but the Linux server options are few, poorly supported and therefore would require a lot more maintenance effort.
The wind generator is still on the list of things to do. But at this location our average yearly wind speed is quite low. I don’t want to put anything up that would annoy the neighbours.
A couple more solar panels and larger battery capacity would be nice. But to make best use of this would require a smarter controller. Maybe one day when I have more time to sort it out.