Potential energy stored in a spring or in the position of a heavy object is like money in the bank — its value is not likely to change. Energy stored as gas pressure is like money invested in the stock market. The value varies with temperature. The inefficiency of the device used to convert pressure to potential energy is like the stock-broker's commission. Buy low, sell high, but be sure the commissions don't wipe out the gains.
Our whole energy problem arises from a general ignorance of the above facts. Everyone knows we are just 20 years away from getting limitless energy from fusion. That's been true for the past 50 years. The energy available from fusion is like money in Uncle Bob's bank account — he might leave it to us, but then, he might not. In the mean time, the sun is a working fusion reactor, and the neutrons it produces stay far away from us. Air in a sealed tank gains and loses energy as it warms and cools. We can play that market while waiting for Uncle Bob to die.
One approach is day-trading. Convert your potential energy to compressed air just before dawn, when the most molecules of gas can be squeezed into a container by a given amount of potential energy. At the daily high temperature in the afternoon, sell some of the high-pressure air to customers. (They might use it to power their cars). Then convert the rest back to the same potential energy you started with, and wait for the next low temperature.
Another approach is to sell on the uptick and buy on the down-tick. Extremely low commissions and high volume are necessary, but think of the profits! The volume is there — every day much more energy arrives from the sun and is re-radiated back to space than humans use in a year. Just capture a tiny fraction of it using properly designed machines spread out over large areas of land, and there is no need to burn fossil fuel.
As you might have noticed, I have been thinking a long time about various ways of capturing energy from ambient temperatures. Very recently I thought of an approach that seems much more promising than anything I have encountered before. A machine using this approach does not operate between two heat reservoirs. Instead, it accepts heat from and rejects heat to its environment, either at daily lows and highs, or whenever the environment's temperature changes.
A day-trading setup might involve some concrete cylinders the size of grain silos for storing potential energy in the form of pumped water, and some high-pressure gas tanks. To convert stored water to high-pressure air, start with a large chamber of atmospheric pressure air connected to the water silos. Open a valve to the lowest silo, and let the water flow in until it stops. Then close that valve and open one to the next higher silo. Continue until the air in the chamber is all squeezed into the high-pressure tanks connected to the top of the chamber.
In the afternoon, the tanks are warmer and the pressure is higher. Transfer some of the air to storage tanks for later sale to customers, but you have to save some to "pay the broker". You need a little more pressure than you started with in the morning to get the water back to where it was. Reverse the morning process: open the valve to the highest silo, and let the water in the pressure chamber flow up into the silo until it stops. Then close that valve and move to the next. When you get to the last silo, you should have enough pressure left to empty the pressure chamber of water, leaving the water levels where they were and the chamber full of air at atmospheric pressure. It will help to have a lot of copper heat-transfer vanes inside and outside the compression chamber, and probably heat pipes too, so that the gas temperature stays at ambient. The process should be done slowly, over an hour or two, the water pipes have to be big so there is no friction losses, and you need enough different level silos so that even with big wide-open water pipes, the water moves without turbulence.
Obviously this scheme works. The thermodynamics is sound. The question is, do you make a profit after paying for the equipment? I don't know. But if we don't find some way to replace fossil fuels, a lot of people are going to suffer.
The other scheme (frequent "buying and selling") avoids working with high-pressure air and massive equipment, but doesn't get a daily harvest. I imagine a cheap device mass produced and distributed far and wide over thousands of acres of open fields, or deserts. After a week or two, a harvester moves slowly over the fields, collecting stored potential energy from the devices but leaving them in place to collect more. Each device includes a pressure chamber with a good thermal connection to ambient temperature, and some means for storing potential energy.
To be vivid and a little cute, imagine that the fields are filled with posts several meters tall, and the devices store potential energy by climbing the posts. The harvester lowers each machine back to the ground while capturing the energy. So, how can a heavy machine lift itself up a pole using nothing but ambient temperature variations? When the price (temperature) is low, the machine buys gas pressure in exchange for potential energy, lowering itself a little down the pole. When the temperature rises enough, the machine sells the gas pressure for a boost up the pole, ending up a little higher than it started even after paying the broker.
There are endless possible variations on the theme, and local conditions would dictate adjustments. If there is a lot of sunshine, the machines might warm up their working gas with sunlight, store the energy, and then cool the gas by sending the heat into the ground. This is more like an ordinary heat engine, but during the night, the machine can still operate whenever a big enough temperature variation happens.
A vast quantity of energy comes and goes every day. We have been using fossil fuels because they were there, and we didn't see anything wrong with it, and we didn't have anything better immediately available. Now we understand that we can't keep using fossil fuels without big unintended effects. All we need to do is capture a tiny fraction of the solar energy the earth blocks, and delay its return to space by a day or two.
Interested parties are invited to join the Renewable Energy Design Wikia (formerly Wikicity) at renewableenergy.wikia.com and work out the details.