A simple solar powered heat engine

In this project we are going to take a heat engine that is normally driven by the evaporation of water, and convert it to solar power.

The "Happy Drinking Bird" is a popular toy often found in science museum gift shops (and, less expensively, in our catalog).

While this is a wonderful little heat engine, to keep it working you have to fill the water cup every day. (We will explain how the toy works a little later, in the section called "How does it do that?").

In our modification, we will eliminate the water, and use the heat of the sun to power the bird. Now the bird can sit in a window sill, and bob as long as the sun shines through the window.

What you need

  • One Happy Drinking Bird
  • One cup of boiling hot water
  • Some black paint
  • Some reflective silver paint (white paint will also work)
  • A sharp knife
Step 1

The first step is to remove the feather tail from the bird. You can do this by scraping the glue off with the sharp knife, but it comes off more easily if the bottom of the bird is first soaked in the boiling hot water for a minute.

No tail

Next, we remove the bird's hat and the fuzzy coating on the head.

Step 2

To do this, we soak the head in the hot water for a few minutes to soften the glue, and then scrape gently with the sharp knife until all the fuzz comes off, along with the hat.

Step 3

Now we paint the bottom half of the bird black.

Black paint

Then we paint the top half of the bird silver.

Silver paint

Then we very carefully put the bird into its stand to let the paint dry. Be careful not to get any paint on yourself, or on the stand.


When the paint is dry, put the birds out into the sun. You may have to adjust the balance point of the bird to get it to bob properly. You can slide the metal pivot up and down the glass tube to find the right balance point. If it is too high, the bird will not tip. If it is too low, the bird will tip completely over, and not be able to return upright.

Bobbing birds

How does it do that?

Before we discuss our solar power modification, it will help to understand how the toy normally works.

The Happy Drinking Bird is made of glass, with a bulb at the bottom, full of a liquid called methylene chloride. A glass tube extends down into the liquid, almost to the bottom of the bulb. At the top of the glass tube is another bulb, forming the birds head. This bulb is coated with fuzz, which holds water when it is wet.

The toy starts working when you soak the bird's head in water. The water evaporates, causing the head to cool. This cools the methylene chloride vapor in the head, creating a small partial vacuum.

The vacuum draws a column of colored methylene chloride liquid up a tube. As the column of liquid rises, the bird starts to tip its head down, towards a cup of water.

Eventually, the column is high enough to tip the head completely into the water. This makes the tube horizontal, allowing the pressure to equalize in both ends of the bird. This causes the mythylene chloride to fall back to the bottom, and the bird rights itself, ready to repeat the whole process.

Methylene chloride is made of a central carbon atom, to which is attached two hydrogen atoms, and two chlorine atoms. It is a very simple molecule that is just barely liquid at room temperature. It has a boiling point of 39.7° Celsius (103.5° Fahrenheit). Because of this, it evaporates very easily, but it is non-flammable.

An important thing about methylene chloride is that it has a high vapor pressure. Molecules at the surface of a liquid in a closed container evaporate and raise the pressure in the container until the pressure is so high that the number of molecules leaving the surface is equal to the number of molecules being forced back into the liquid by the pressure.

For water at room temperature, the vapor pressure is 3 kilopascals. Methylene chloride's vapor pressure is 46 kilopascals.

Vapor pressure changes as the temperature changes. This is especially true for substances with high vapor pressures. The vapor pressure goes up as the temperature goes up.

The wet head of the bird can be several degrees cooler than the bulb at the bottom. The vapor pressure at the bottom is higher than the vapor pressure in the cooler head. That pressure pushes the liquid up the tube.

When the bird tips, and the tube is horizontal, the vapor in the bottom and the vapor in the top are connected by the tube, since the tube is no longer blocked by liquid. The pressures are now equal, since the chambers are connected. The liquid is now free to run downhill into the bottom again, and the bird tips up.

The Happy Drinking Bird can be used to indicate the relative humidity. When the humidity is low, the bird will dip more times per minute.

The solar modification

In our modification, the bottom of the bird is painted black. Sunlight that hits black paint is absorbed, and the paint warms up.

The top of the bird is painted with a reflective silver or white paint. This paint does not absorb the sunlight, so it stays cooler than the black bottom.

So, we have a warm bottom, and a cool top, just like we did when the bird's head was kept damp.