Propane in the U.S. is a mixture of approximately 85% propane and 15% butane; small amounts of pentane and isobutane are also incorporated. Outside the U.S. and Canada, the mixture can vary greatly from 80/20 to 30/70 propane to butane. Propane has a boiling point of -42°C (-44°F) at atmospheric pressure; therefore, when kept in a sealed vessel, propane has a vapor pressure that moves depending on outside temperature. For example, at 70°F, the vapor pressure of propane is 127 psi and of butane is 17 psi, so the vapor pressure is a weighted average of the two constituents. As the outside temperature drops, the vapor pressure also drops. Depending on where the fuel is being used, it may require some sort of active vaporization device to ensure that the fuel is in gaseous form when used. In liquid form, propane has a heat content of approximately 91,500 Btu/gal. In gaseous form, it has a heat content of approximately 2500 Btu/cu3 (natural gas has a heat content of approximately 1000 Btu/cu3).

Propane is gathered and processed from both natural gas and oil wells throughout the world. It is heavier than natural gas (96 to 98% methane) and than air, so in gaseous form leaks remain explosive for a longer period of time than natural gas because they take longer to dissipate. Propane can also displace oxygen, so there is danger of asphyxiation if care is not taken. For this reason, the U.S. government requires that propane be artificially odorized.

The primary market for propane is the residential and small business heating market, so propane prices are directly related to winter temperatures. During winter of the year 2000, the wholesale market price of propane was approximately $0.55/gal, with retail ranging from $0.65 to $1.35 per gallon. The price outlook for propane is flat for the next three to five years. Due to the four warmest winters on record in recent history, propane supplies are at an all-time high, and a very cold winter over much of the U.S. would be required to move prices a noticeable amount. Compared to natural gas, which usually costs U.S. residential consumers $3.00 to 7.00/mmBtu, propane costs about $7.50/mmBtu at current prices.

Propane is readily available in bulk and retail in all 50 United States and most areas of Canada and Mexico. In other markets, countries that are net exporters of petroleum products usually have an ample supply of propane, while those countries that are net importers tend to use very little and have little infrastructure for its use and delivery.

The propane industry is one of the best suited to deliver remote Btus to users who need a clean-burning fuel for a reasonable price. The average setup for basic home delivery costs about $500 and is usually leased to the home or business owner. Most power generation applications will require a larger tank, roughly 1000 to 2000 gallons at a price of $3000 to $6000. The tank must be certified and tested to the specifications required by local, state, and federal laws. These rules are fairly standard, and propane tanks are usually tested to two or three times their operating pressure, but, generally, the operating pressure will not exceed 250 psi. Each tank is equipped with a hydrostatic relief valve that will not allow the interior gas pressure to exceed the pre-set point. Propane installations are very safe, and the propane industry has a very good safety record. When these setups are used to fuel a DG installation, it is recommended that a propane-fired vaporizer be added to insure that the propane flows to the machinery in gaseous form. In colder climates, the temperature may be such that the propane will not gasify fast enough to supply the generator with sufficient gaseous propane.

Depending on electricity requirements, including location and availability of fuel, propane can and will be the fuel of choice in many situations. Since the propane industry is so well suited for fast and efficient delivery of remote fuel, propane will always be considered a good alternative for remote power applications where natural gas pipelines do not reach. Another attractive application of propane is for use as a backup power fuel. When the main purpose of a DG project is to provide power reliability, a higher confidence factor can be used for a fuel that is actually on-site, as opposed to natural gas, which is dependent on a system that could suffer disruption during a natural disaster.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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