# Homework Help: Question about dc to ac circuits in solar panels?

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1. Jan 24, 2015

### 21joanna12

1. The problem statement, all variables and given/known data
It can be shown that a square wave can be represented by a sum of sine waves as given by the formula below

v = v0sinωot + v1sinω1t + v2sinω2t + v3sinω3t +.....

Solar cells have an output that is dc. The pd across a cell depends on the incident radiation and the efficiency of the solar cells – Hubble telescope cells have an efficiency of about 30% in full sunlight. Solar cells for use on buildings etc. normally have an efficiency of about 15%. The dc power from the solar cells has to be transmitted to the mains which is 240V ac. At your disposal you have electronic black boxes. A possible solution is sketched out in Figure 2.1

It is question number 2.

https://www.physicsforums.com/file:///page4image6144 [Broken]
What is the black box for? Suggest components for the black box.

Some electrical energy may be lost as heat. How can you maximise the efficiency of the electrical energy transfer from the solar panels to the 240 V ac of the mains?

Explain why electrical energy is transferred by the grid using three or four wires.

It has been suggested that one way of storing electrical energy would be to store it in a rotating flywheel. A simple flywheel should store electrical energy from a 1MW power supply for 20 minutes. Suggest a design for this flywheel.

2. Relevant equations

3. The attempt at a solution

F
or the first part, I am guessing the black box is to convert the dc to ac, and also to ensure you have a 240V supply even though sunlight intensity changes. To change dc to ac, I think you would have a gate that periodically turns the cd current on/off at a frequency of 50Hz, and a combination of an inductor and capacitor to turn the dc into ac. Although I am not sure how you could ensure you always have a 240 V output if you have a variable voltage input. Is there a component that deals with this?

With regards to the efficiency, I am guessing that it is just about having a circuit with as little resistance as possible to reduce the heating effect in the wires. For the same reason, you use a greater number of thinner cables in the grid- less power wasted, but this time it is because the greater wire resistance means that a smaller current flows, and the power lost is proportional to I^2 so you reduce power loss.

I have no idea for the last part. Could I have a hint?

Unfortunately there are no answers to this...

Last edited by a moderator: May 7, 2017
2. Jan 24, 2015

### rude man

Link does not work on my pc.

3. Jan 24, 2015

### Staff: Mentor

The link worked for me. Here's the figure:

4. Jan 24, 2015

### rude man

Thanks g. You can take it from here.
r m

5. Jan 24, 2015

### Staff: Mentor

I believe you need to think in terms of smaller "black boxes" within the black box. Rather than trying to identify particular elecronic components or mechanical devices, think in terms of the functions that might be required.

You've identified DC to AC conversion. That's one function. You've touched on voltage regulation, ensuring 240 V AC output. Will any 240 V AC voltage waveform be adequate or do you need to worry about matching the grid's waveform?

Handling a varying energy supply from the solar cells is another area. What do you do about night time and cloudy days (any ideas?)? If you can't supply power to the grid due to lack of input energy, what decides when to disconnect from the grid? Should it be automatic?
You can research DC to AC conversion and how efficiency is dealt with. There are several ways to convert DC to AC, everything from DC motors driving AC generators to electronic switching inverters. Modern methods tend to favor higher frequency intermediate steps to reduce component size (transformers) and heating losses. Those uninterruptible power supplies for keeping your home PC running (for at an hour or so) during power failures come to mind as an example. You might look at some vendor websites to see what they put in their boxes.
A little research on the web for flywheel energy storage should get you started. What form of energy does a flywheel store?

6. Jan 25, 2015

### 21joanna12

Thank you for your reply! I am still having some difficulty with two parts:

I am still unsure as to how you can make a variable input pd always 240V. I don't think it is possible because of conservation of energy, but is there some component that does this?

I have researched flywheels and am trying to design my own, which is consisting of an electric motor connected to the ac supply and a disc attached to the motor. I know the disc will store rotational kinetic energy, but I cannot figure out how the electric motor will work in conjunction with the disc. I need to work out the maximum speed that the electric motor will reach, but theoretically there should always be a torque on the motor so it's angular velocity should keep increasing as long as current runs through the motor. I am sure that back emf has something to do with this, but I can't figure out what the maximum back emf will be, and how it will affect the speed of rotation...

7. Jan 25, 2015

### Staff: Mentor

It's called a regulator in general. The objective of the system is to supply power to the grid. Electric power is the combination of voltage and current, $P = VI$. Power is an energy term, being the amount of energy used or delivered per unit time. So when you worry about conservation of energy limiting things, it's the balance between the energy available from the source (sunlight) versus the energy delivered to the grid.

If the power available from sunlight varies, then the power you can deliver directly from that source will vary too. You can hold the voltage constant at the expense of current delivered.
A DC motor might be a better option. They offer variable speed and they can function as both motor and generator. Efficiency is increased when you skip conversion steps (AC ↔ DC). AC motors can have variable speeds but that is usually at the expense of efficiency, the control circuits can be wasteful of energy. If you want to use an AC motor, it might be worthwhile considering mechanical speed control mechanisms (Clutch, gearing, etc.. There are many variations).

While the powering of the flywheel is important, I think the intent of the question is to get you to consider practical limits on the mass, materials, and rotational speeds of the flywheel itself.

8. Jan 25, 2015

### Staff: Mentor

They probably want you to think about the cost-effective design of a high-energy flywheel for this application, including materials and safety considerations.