Week 3 - Electrical Power Subsystem

Problem 3)

In order for a satellite to successfully collect solar energy, it must be in view of the sun.  Since planetary motion is cyclical, this occurs periodically and poses a challenge to the EPS.  Now we need to know how much time a satellite will be in the sun and how long it will need to be powered by energy stored in the battery.  It is advantageous that the sun is a free and abundant energy source, and that solar panels are lightweight, flat and therefore easy to incorporate.

Problem 4)

A way to use less power on the dark side is to run systems in a conservation mode.  Systems function normally while in view of the sun and the battery stores excess energy to be used on the dark side.

Problem 3:

Consumer electronics, such as cell phones, iPods, and laptop computers resemble satellites in the area of electrical power generation and distribution. For starters, they will have some means of generating electricity. This could be accomplished directly by way of a battery or by taking electricity from a an AC source such as a wall outlet. AC adapters for computers usually have built in converters and/or transformers that adjust the voltage or current levels to those needed by the various components. Regulation will also be important, and this may be in the form of a surge protector or circuit breaker. These will ensure that too much current or voltage does not enter the system. As previously mentioned, batteries will play the role of storage. For individual circuits, capacitors store energy that is released in a manner appropriate for the component incorporating them. As far as distribution, internal wiring and circuitry meet these ends.

Consumer electronics could potentially be operated with a solar power source. Some already do, in fact. Wristwatches and calculators have utilized small solar cells for years. Larger devices could be modified to use solar power, but this may not be realistic. Solar cells are very inefficient (~20-30% at most) and so more cells would be needed to create the power needed for a larger device. At a certain point this would become unwieldy and too bulky to be practical for a device that is designed to be ultra-portable and compact such as a cell phone or iPod. It is difficult to imagine a scenario in which solar cells could be used independent of batteries. Solar power isn't effective at night or on cloudy days, and so some means of power storage would be required.

Problem 4:

Power companies transfer power at high voltage because of the distances involved in power transmission. Because large-scale power production is normally localized in large power plants, power lines are required to deliver this to customers far away. Any conductor used for the wire will invariably have some inherent resistivity that will cause resistance and power losses along these lines. The longer the power has to travel, the larger the losses. In order to minimize this phenomena without overloading the current restrictions of the wires, companies can instead increase the voltage instead. Power is defined as voltage multiplied by current (P=VI or I^2R). In this way, higher power levels can be achieved with higher current or higher voltage levels, or both. Higher voltage is the most realistic and effective way of accomplishing this.

Problem 3)

EPS of consumer electronics share similar parameters with EPS of small satellites.  They both have avg. electrical power and peak electrical power requirements, and mission life to consider.  A cell phone may require smaller capacity batteries and alternate redundancy design than a satellite.  A cell phone EPS doesn't consider orbital parameters.  Consumer electronics and  small satellites are similar in that they both utilize GPS sensors.  Both involve similar functions such as power source, energy storage, power distribution, and power regulation.

In order to utilize solar energy, photovoltaic cells need exposure to sunlight.  A cell phone could be powered by solar energy if it had efficient exposure to sunlight.  It may also be convenient to power other electronics (watches, and calculators) this way.  An outlet recharge would probably work better for a cell phone.

Problem 4)

Power companies transfer power over power lines at high voltages because as voltage increases, current decrease (with constant resistance).  It is beneficial to operate at a lower current, I, because power lost across miles of power lines, is P = I^2R.  This means that any raise in current will increase power loss exponentially, and power companies prefer high energy transfer efficiency.

3) Discuss how a consumer electronics product, i.e. a cell phone or laptop, has an EPS similar to that of a small satellite. Could we also power these products with Solar energy?

Answer: In a small satellite, there is a power source, energy storage, power distribution, power regulation and control, and power conversion. In both cell phones and laptops, the power source is chemical energy that is to be converted into electrical energy by using electrochemical cells in an electrical battery. The battery also acts as a storage device, which can be recharged through an outlet or now, a micro-USB connector which allows a cell phone's battery to be charged by connecting it directly to a computer. Power is distributed through multiple PCBs (printed circuit boards) and controlled by voltage regulators, which help increase battery life. For example, voltage regulators will step down the voltage between the battery and sub-circuits that require less supply voltage, and step up the voltage for the sub-circuits that require more.

Many companies are testing solar cells to supply power to cell phones and laptops, but since the efficiency of solar cells are not at 100% or close to that amount at the moment, it is not yet widely used by consumers. The use of solar cells would also only work if the charger for the battery utilized them and charged the battery outside, since laptops and cell phones used indoors would not be able to use the sunlight for power. Currently, solar power for laptops and cell phones would only be effective if the electronic device did not depend solely on solar power and if it acts just as a supplement to the existing charger.

4) Why do power companies transfer power over power lines at high voltages?

Answer: Since P=VI and V=IR, substituting V with IR, P=I²R, which describes the power loss due to resistance. Over long power lines that stretch for miles, the resistance is much higher, which causes the power loss to be greater and the supplied power to not be at its maximum efficiency when it arrives to its destination. This is why power companies transfer power at high voltages so that they can reduce the I²R losses and have optimal power supplied to the customer effectively.

1) Nickel Cadmium (NiCd) batteries are a popular choice to use in satellites. Look up a data sheet for D cell NiCd batteries at digikey.com. Based on the data sheet, what is the normal operating voltage and capacity of a cell? Assuming our EPS needs to provide 5VDC, how many cells would we need to provide this voltage? What can we do to get the voltage we require?

Answer: The normal operating voltage for D cell NiCd batteries is 1.2V and the capacity of a cell is 4000mAh - 5000mAh.

We'll need 5 cells in order to provide the 5VDC. To get the voltage required, we can wire the batteries in series to obtain a total of 6VDC, then we can create a circuit with a 1ohm resistor and a 5ohm resistor (or any other two resistors with a 1:5 ratio) and connect the load in parallel to the 5ohm resistor (or the respective resistor if using different ones). Groups of resistors can also be used to obtain the 1:5 ratio.

2) Create a simple block diagram for a small satellite EPS with the parts we've discussed. For example, one block for Solar cells, one for batteries, and another for voltage regulation and so on...refer to the fundamental power functions we discussed.

 3) Consumer electronics often have very complex power requirements due to the variety of components in them.  For example, a cell phone relies on its antenna for transmission, a keypad for input, and microphone and speaker for audio input/output.  All of these components have different power requirements, and thus require a EPS that can meet these requirements.  It would need a voltage regulator to dole out proper voltage requirements for the different components, a battery and capacitors to store energy, wires to transfer the power, among other things.  It would also need to have redundancy built in because without redundancy, the electronics become too reliant on one component of the EPS and would thus become quite fragile.  Consumers will not appreciate this, and business will probably hurt as a result.  A solar energy source could power these systems, but at the current state, they are not efficient enough to overcome charging these electronics through outlets.  Most uses of these electronics will bring the electronic near outlets, which is a faster and more convenient way to charge for most consumers. 

4) Power companies transfer power over lines at high voltages because they need to transfer the power over a large distance (the entire city/section of the city).  At low voltage/high current, there is a significant amount of power consumption (P=I²R; the lower the I, the less power consumption), thus a high voltage/low current method is more desirable.