Deployment Guide/Power Infrastructure
6. Power
The OLPC laptop is the most power-efficient laptop ever built, but it still needs a source of electrical power. In ebook mode, it runs at less than 2 Watts. Average power consumption is approximately 5 Watts.
We have been working with a number of alternative power systems, including solar, human-power, etc. Currently we have 5-, 7-, and 10-Watt solar-panel solutions that are quite attractive for individual laptops (5 Watts is a supplement for extending the battery life while out in the sun); we are also developing a solar-panel multi-battery classroom charger—a small number of prototype units will be available soon. We have still not achieved our goal of a cost-effective school server that can be powered off of the grid. Please take this into consideration when doing your site preparation. Another consideration is energy storage: if you need to run when the sun does not shine, you'll need a battery system for the school and ample power to both charge the battery and run the school server at the same time.
The following worksheet can be used to estimate the power requirements for each school. (The Watt-hours are dependent on how long the children are in school, whether or not they are charging their batteries while they are working, and how many hours per day the school server and connectivity are operational.)
| Number of units | avg. Watts | Total power required | |
|---|---|---|---|
| Laptops | 100 | 5 (15 peak) | 500 Watts |
| School Server | 1 | 20 (24 peak) | 20 Watts |
| Connectivity
(e.g., Vsat modem) |
1 | 50 | 50 Watts |
| Total Power | 570 Watts |
The total energy required to operate 100 laptops and a school server over an eight-hour period is approximately 570 Watts times 8 hours or 4560 Watt-hours. If, for example, you wanted to generate and store this energy over the course of a two-hour period, you'd need roughly 11400 Watts generation capacity feeding a battery system with adequate storage capacity, assuming 80% efficiency. (As a datapoint, Wikipedia gives the lead-acid battery a 70-92% efficiency.)
(An additional consideration is the means to provide power to individual laptops in the classroom. Due to physical-design constraints, our power adapters are oriented to be used with power strips that have sockets are oriented parallel to the length of the strip, e.g., = = = =. Power strips with a perpendicular orientation, e.g., || || || ||, are inefficient as adapters cannot take advantage of every outlet. At least one manufacturer is developing a system that directly incorporates the power supply for up to eight laptops in a single unit, eliminating the need for individual power adapters. This is still in early prototype stage, but it looks promising.)
Some questions you should consider if you are planning to make extensive use of solar energy:
- Is this a no power grid environment? Is solar the only power source?
- How much sun will the deployment area have? How long and often do you have clouds and rain? How long during the day do you have bright, strong, hot sunshine?
- Do you have any data on the average daily solar irradiance?
- Do you require a school server (or other equipment) that has to be solar powered in addition to the laptops?
- What is the physical set up of the places where the laptops will be used? Will it be indoors or somewhere sun is available?
- How many of the laptops will be in use simultaneously (per site) and how many need to be recharged at any one time?
- What is the planned usage pattern during the day? Will they be taken home and used so that in the mornings the battery is drained or low?
The 10W panel at full output will fully charge a drained battery in just under three hours if the laptop is turned off; if you can take advantage of the peak solar hours (11:00 to 14:00), then you should be able to keep the laptops charged, but this means not using them during that period.
Note: in the solar energy business we usually say it does not matter at all WHEN you use the electricity. All that matters is how much energy is collected and how much energy is used. So unless the charging capability is actually disabled when the XO is being used, I would say that the time of use is completely irrelevant.
If the laptop is running, then full sun with the 10W panel will provide enough average power to run the laptop and slowly charge the battery (about six hours). If you don't have full sun, then you will break even or slowly drain the battery.
