will have increased, as also the temperature of the water and steam. It is this increase of temperature that constitutes the storage of energy, and with the same increase of temperature above the normal working temperature, the amount of energy stored will be proportional to the quantity of water heated. Since the quantity of water usually employed in our boilers is comparatively small, the story of energy in them within the limits of safe pressure is, therefore, limited. Mr. Halpin, however, has conceived the plan of connecting the boilers to large iron storage tanks filled with water which is brought to the same temperature and pressure as the boiler. It is a condition of any system of heat storage for central stations that the energy stored should be recoverable whenever and at any rate of supply required. Superheated water fulfills this condition, for if the pressure is reduced steam is generated instantly and in controllable amount. This has given rise to three methods of procedure, which he has designated respectively "steam storage,' "feed storage" and "combined feed and steam storage."

The first of these was very fully described by Professor George Forbes at the St. Louis Convention. The other two are later developments.

In the steam storage system Mr. Halpin employs only sufficient boilers to supply the mean demand and storage tanks tanks sufficient to supply the maximum demand. These latter, not being subjected to the fire, will suffer but little deterioration, and being simple iron reservoirs of sufficient strength to withstand the pressures required are comparatively inexpensive to install. The boilers working continuously at their most economical rate have their excess of energy during light load stored up in the water of the tank, from which it may be drawn at will during heavy load.

He proposes that the boilers and tanks shall work under a pressure of 265 pounds per square inch when fully charged, which corresponds to a temperature of 406 degrees Fahrenheit. He proposes that the engines

be worked at 130 pounds per square inch, which corresponds to 347 degrees Fahrenheit. Fahrenheit. The total available heat stored when the reservoirs are fully charged is the difference of the total heat of the water

at 406 degrees and at 347 degrees Fahrenheit, or that due to a range of temperature of fifty-nine degrees. Every pound of water falling in temperature through that range will yield sixty-one thermal units of heat. The total heat required to generate a pound of steam at 130 pounds per square inch from water at 347 degrees is 868.8 thermal units. Consequently, fourteen and one-quarter pounds of water falling in temperature from 407 degrees to 347 degrees will yield one pound of steam. To allow for radiation loss and imperfect working, this may be taken at sixteen pounds of water per pound of steam. The steam consumption. per effective horse-power may be taken at eighteen pounds per hour in condensing and twenty-five pounds per hour in non-condensing engines. The storage room per effective horse-power by this method would, therefore, be 1818 4.06 cubic feet for condensing and 1635 6.4 cubic feet for non-condensing

engines.

62.5

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Gas storage, assuming that illuminating gas is used, would require about twenty cubic feet of storage room per effective horse-power hour stored, and if ordinary fuel gas were stored it would require about four times this capacity.

In water storage 317 cubic feet would be required at an elevation of 100 feet to store one horse-power hour, so that we see that of the three methods of storing energy the thermal method is by far the most economical of space.

In the steam storage method the boiler is completely filled with water and the storage tank nearly So. The two are in free communication by means of pipes, and a constant circulation of water is maintained between the two, but the steam for the engines is taken only from the top of the storage tank through a reducing valve.

In the feed storage system, the excess of energy during light load is stored in the tanks as before, but the boilers are not completely filled. In this system the steam is taken exclusively from the boilers, the superheated water of the storage tanks being used during heavy load as feed water to the boilers.

The third method, as the name would imply, is a combination of these two. In the "combined" feed and steam storage system the pressure in boiler and storage tank is equalized by connecting the steam spaces in both by pipe, and the steam for the engines is, therefore, taken from both. In other words, they work in parallel.

An incidental advantage of thermal storage is the purity of the water supplied to the boilers. Since the latter derive their feed water from the hot reservoirs, it will have deposited its impurities where they can do no harm. The only unavoidable losses that need be considered in connection with thermal storage are those due to radiation from the reservoirs. This can be provided against by proper lagging so as to be insignificant, and can be still further reduced if there be a number of reservoirs. If, for instance, there are four, each will radiate from three-quarters of its surface towards the other three, and only from onequarter of its surface into space.

STEAM STORAGE SYSTEM.

As illustrating the modus operandi and cost of the system of "steam storage,' storage," as it is called, I quote from a report by Mr. Wm. Schonheyder, who, taking the load line of the Berlin electric lighting station as a basis, has figured out the relative costs of operation with and without a steam storage system. Fig. 1 is