solar white-paper part3 10

Solar Energy and Steam for Small Turbines and Engines Junk Yard Mechanics at its Best Series

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If you can make steam, you can make electricity.

MAKING STEAM FROM

SOLAR ENERGY

for

Small Steam Turbines and Engines

Part 3

Steam Safety

And

Introduction to the Small Steam Turbine System

February 6, 2010

By

Robert Saunders

[email protected]

_____________________________________________________________________________________Copyright 2008, 2009, 2010 Robert D. Saunders of 23


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Quotation:

“Steam is no stronger now than it was a hundred years ago but it is put to better use”

Ralph Waldo Emerson quotes (American Poet, Lecturer and Essayist, 1803-1882)

http://thinkexist.com/quotes/with/keyword/steam/

Preface

This paper is related to an article entitled Slotted Disc Turbine Concepts, originally published on the following website:

www.green-trust.org/steamturbine/steamturbine.htm

Caution. When steam is contained, as in a boiler or vessel, it can be very powerful but tame, and when it gets loose it can become very very dangerous, like a proverbial dragon.

Anyone familiar with, or who has ever worked in, a power plant has a very great respect for two things; high voltage electricity and high pressure superheated steam. When a leak develops in a superheated part of a boiler the resulting sound can be deafening, but the location of the leak may not be easily discernable because superheated steam is not visible. It can be difficult to know exactly where the leak is located.

At one time, when a mechanic needed to locate a leak he would approach the area waving a wooden broom handle in front of him and when the handle got cut in half he knew he had located the leak. Such a leak can begin with a pin-hole in a steel housing that can quickly erode into a much larger hole, and that can be very dangerous.

The following articles are referenced here for readers who have not yet read Parts 1 and 2 of this series of articles:

www.green-trust.org/steamturbine/Solar_White%20Paper_Part%201.pdf

www.green-trust.org/steamturbine/Solar_White%20Paper_Part2_03.pdf


http://www.green-trust.org/steamturbine/Solar_White%20Paper_Part2_03.pdf

http://www.green-trust.org/steamturbine/Solar_White%20Paper_Part%201.pdf

http://www.green-trust.org/steamturbine/steamturbine.htm

http://thinkexist.com/quotes/with/keyword/steam/


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1 Introduction..................................................................................................41.1 Making Hot Water..................................................................................................................................51.2 Making and Using Steam........................................................................................................................5

2 Steam safety.................................................................................................52.1 Boilers.....................................................................................................................................................5

2.1.1 The Mono-tube, Multi-tube and the Pressure Vessel Boiler...........................................................62.1.2 Steam Radiators, Home Heating and CHP Systems........................................................................72.1.3 Boiler Safety and Codes. ...............................................................................................................7

2.2 Commercial Off-The-Shelf (COTS) Components..................................................................................8

3 Introduction to the Small Steam Turbine System........................................83.1 The Closed Loop Steam Turbine System...............................................................................................9

3.1.1 The Cold Water Supply.................................................................................................................103.1.2 The Hot Water Supply Tank..........................................................................................................103.1.3 The Boiler and the Steam Generator.............................................................................................11

3.1.3.1 Dangers from a Boiler Failure................................................................................................113.1.3.2 Boiler Water Level.................................................................................................................11

3.1.4 The Super-heater............................................................................................................................123.1.5 The Boiler Feed Pump...................................................................................................................123.1.6 The Pressure Relief Valve (PRV)..................................................................................................123.1.7 The Turbine...................................................................................................................................13

3.1.7.1 Estimating Turbine Thermal Efficiency.................................................................................133.1.7.2 Other Steam Turbine Efficiency Concepts.............................................................................14

3.1.8 The Condenser...............................................................................................................................143.1.8.1 Using the Exhaust Heat..........................................................................................................15

3.1.9 The Steam By-Pass Valve.............................................................................................................153.1.9.1 The Water By-Pass Valve.......................................................................................................15

3.1.10 The Purge Control Valve.............................................................................................................163.1.11 The Alternator and Generator Head............................................................................................163.1.12 The Controls................................................................................................................................183.1.13 Controls and Safety Devices for Small Systems.........................................................................18

4 An Example: Making Steam for 10 kW.....................................................194.1.1 Calculate Water Rate and Heat Needs Using 25% Thermal Efficiency........................................20

4.1.1.1 Physical Size Estimate for a 10 kW Solar Collector..............................................................214.1.1.2 Exhaust Steam Conditions......................................................................................................21

4.1.2 Calculate Water Rate and Heat Needs Using 100% Thermal Efficiency......................................22

5 Summary of Part 3......................................................................................235.1 Preview of Part 4..................................................................................................................................235.2 Notes to the reader:...............................................................................................................................23



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1 Introduction.

The purpose of this paper is to offer useful information for those readers considering or planning to build a solar steam driven turbo-generator using a satellite parabolic reflector or a series of trough reflectors. In review, Parts 1 and 2 provide some concepts about heat and a few simple calculations to estimate the amount of useful energy that can be collected from the sun and converted to steam. To determine the needs of the reader consider and answer these questions:

• How much energy is available from the sun in your latitude?• How much energy do you need?• What happens when the sun doesn’t shine?• How much energy can you collect from the sun?• How long does it take?• How are you going to convert the energy to useful heat or work?• How much will it cost?• How much will you save?

There is a widespread misconception of what solar power is all about and how a solar collector collects and concentrates energy. The first rule to remember is:

Concentrating solar heat does not increase the amount of energy collected.

It only squeezes that energy into a smaller space, as may be indicated by a rise in temperature. This can be demonstrated when a magnifying glass is used to ignite some flammable material. In this case, radiated energy from the sun is concentrated by focusing the sun’s rays on a small spot where it is absorbed and converted to heat energy, thus raising the temperature of the material receiving the energy to the point where it ignites. The amount of energy available across the surface of a parabolic collector is fixed at any point in time and does not change regardless of the resulting temperature at the focal point of the collector.

As an example, consider the following. The power density of solar energy at certain locations on the surface of the earth is one kilowatt per square meter (1 kW/m^2). A round three-meter parabolic reflector has a cross section area of about 7 square meters facing the sun and therefore can collect about seven kilowatts (seven kilowatt-hours per hour, 7kWh/h), but only intermittently at best. Solar energy is only available when the sun shines, depending on location, season of the year, local weather and time of day. Solar energy is by nature only available on an intermittent basis, but it can be stored, e.g., in a battery or by other means, for use at a later time.



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1.1 Making Hot Water.

If the objective is to heat water, e.g., for domestic purposes, consider the following. In a day of bright sunshine, a 3-meter concentrator might collect 23,884 Btu per hour, or approximately 250,000 Btu per day. That is enough heat to raise the temperature of approximately 300 gallons of water from 40 degrees F to 140 degrees F. Alternatively, less water can be heated to a higher temperature. If stored in an insulated tank, that would be sufficient for most home hot water needs. However, such systems are not very efficient. If the overall efficiency of the system is 50%, the result would be only 150 gallons of hot water. This is still sufficient for many homes and if more water is needed, two concentrators can be connected in parallel. If water at a higher temperature is needed, additional concentrators can be added in series.

1.2 Making and Using Steam.

Making steam to drive a turbine or engine is a bit more complicated than just boiling water. The steam at the intake to the turbine must be superheated, dry, hot enough, and under high enough pressure to be able to convert heat energy to mechanical energy sufficient to overcome the resistance of the turbine or engine load. If steam is made using a single solar concentrator, and used directly to drive a steam turbine or engine with an overall system efficiency of 25%, the maximum output power of the turbine or engine is about 1.75 kilowatts.

Note. The exhaust steam must remain superheated and dry and never reach the saturation level. Wet steam can result in severe damage to a turbine or to a steam engine.

2 Steam safety.

2.1 Boilers.

For purposes of this discussion a boiler is defined as any closed container in which water is boiled. Most boilers use either pressure vessels or tubes in which water is boiled, or a combination of both.

Where a satellite dish is used to collect and concentrate solar energy, a pressure vessel that forms the boiler is located at the focal point of the parabolic surface. In the case of a parabolic trough, a tube running along the middle of the reflector, inside which water is boiled, is the boiler portion of that solar concentrator. In either case, the device at the focal point or foci is commonly referred to as a receiver.

In all cases where a boiler is in operation, there is pressure in the system. The amount of pressure depends on several factors, but temperature is arguably the most important. Also important is the volume of the portion of the boiler that contains the steam. Where



_____________________________________________________________water is boiled to make steam, the combination of the volume, the pressure and the temperature together defines the amount of energy contained in the steam within the boiler. Boilers that make superheated steam require a separate super-heater section to raise the steam temperature.

At the risk of oversimplification and for purposes of this discussion, according to Charles’ Law, the following relationship exists between pressure, volume and temperature for an ideal gas. In a closed vessel with a given volume, V, and a given weight of gas, the absolute pressure of a gas is proportional to the absolute temperature of the gas:

P1/P2 = T1/T2

Where T is the absolute temperature in degrees Fahrenheit and T’ is the temperature as measured in degrees Fahrenheit:

T = T‘ + 460

For more on the Ideal Gas Law see the following website:

en.wikipedia.org/wiki/Ideal_gas_law

The operating size of a boiler is usually measured by its output in terms of lbs/hr, tons/hr, Horsepower, etc. The following website further explores boiler topics:

en.wikipedia.org/wiki/Fire-tube_boiler

Note: Any reader even thinking about building or installing a boiler in a dwelling is urged to check local, county, state and federal regulations before doing anything else. Otherwise a local expert or inspector should be consulted first.

2.1.1 The Mono-tube, Multi-tube and the Pressure Vessel Boiler.

While almost all steam boilers are made up of tubes and headers, it is possible to create a boiler from a single pressurized container. A mono-tube boiler suggests a boiler containing a coil made from a single tube. A multi-tube boiler suggests a number of tubes connected in parallel between plates or headers.

The old fashioned pressure cooker with a weight mounted on its top to act as a pressure relief valve is an example of a boiler consisting of a single pressure vessel. Although the pressure cooker has been widely replaced by the much safer microwave oven, it is still used in some homes because of its efficiency and ease of use.

When a pressure vessel fails, there is often an explosion in which all the contained energy is almost instantaneously released with resultant collateral damage. When a coil or tube



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fails, there is a relatively gradual release of energy and much less chance for an explosion resulting in collateral damage.

Note. For a number of case studies of steam turbine accidents, the reader is encouraged to search google for steam turbine accidents.

2.1.2 Steam Radiators, Home Heating and CHP Systems.

The discussion and links in this section are not directly related to steam turbines, but might be valuable to the reader contemplating building or installing a CHP system.

www.bbc.co.uk/dna/h2g2/A533819

The following white paper discusses several important topics for steam heat applications, including home heating and heat distribution systems.

www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12580

The following website covers more technical details for designing a home heating system using steam for heat distribution.

www.engineeringtoolbox.com/steam-heating-systems-d_474.html

Here is another technical dissertation that readers who have read and understood Part 1 and Part 2 should be able to understand.

chestofbooks.com/architecture/Modern-Buildings-Construction-V3/Chapter-VII-Steam-Heating-Apparatus.html

Also see the following How steam Technology works:

science.howstuffworks.com/steam-technology7.htm

2.1.3 Boiler Safety and Codes.

Because of public safety concerns with the operation of steam boilers, there are local, county, state and federal codes governing the building and operation of boilers. Insurance companies are concerned where boilers exist in properties they cover.

www.edgeta.org/steamsafety.htm

The term mono-tube generally refers to a tube construction configured in the shape of a coil. Designs including a small pressure vessel with a maximum capacity of five gallons may be included for parabolic satellite dish applications. In all cases only low pressure superheated steam should be considered. One reason to limit designs to the use of low-


http://www.edgeta.org/steamsafety.htm

http://chestofbooks.com/architecture/Modern-Buildings-Construction-V3/Chapter-VII-Steam-Heating-Apparatus.html

http://chestofbooks.com/architecture/Modern-Buildings-Construction-V3/Chapter-VII-Steam-Heating-Apparatus.html

http://www.engineeringtoolbox.com/steam-heating-systems-d_474.html

http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12580

http://www.bbc.co.uk/dna/h2g2/A533819


_____________________________________________________________pressure steam is that certain required components are readily available from local hardware stores or from catalogs. The following article consists of several sections and offers the reader more information on commercial boiler and home heating systems.

homerepair.about.com/od/heatingcoolingrepair/ss/trblsht_boiler.htm

2.2 Commercial Off-The-Shelf (COTS) Components.

Two main reasons for choosing off-the-shelf components are cost and availability. This concept and the acronym COTS are related to Department of Defense (DoD) principles of procurement on some programs.

As an example, the standard household hot water heater always has a safety valve or Pressure Relief Valve (PRV) attached to the tank and vented through a hose connection to the floor or to a drain. This common PRV is set to open at 150 psi or at 250 degrees F. Commercial hot water tanks are pressure tested to some higher pressure (generally to 300 psi) to assure an adequate margin of safety. Likewise, for superheated steam at 100 psi, a temperature specification of 600 degrees F is the maximum temperature attainable in the common kitchen oven. Hardware rated at these temperatures and pressures are usually available locally.

steamtraction.farmcollector.com/Steam-Engines/Boiler-Safety.aspx

www.hvacwebtech.com/boilers.htm

3 Introduction to the Small Steam Turbine System.


SolarCollector

ConcentratorFunctions

SteamTurbine / Engine

Functions

ControlFunctions

Figure 1 System Functional Block Diagram

http://www.hvacwebtech.com/boilers.htm


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3.1 The Closed Loop Steam Turbine System.

Figure 2 illustrates a block diagram of a small closed loop steam turbine system. The discussion in this section is mainly limited to small steam systems while larger steam systems are to be discussed in a later article. A distinction is made between a small system with an output equal to or less than 10 kW and a larger system with an output greater than 10 kW. In some circles, these are referred to as micro-turbines.


Figure 2 System Block diagram - Small Steam Turbine

SolarReceiver

PRV#1

Alternator

Cold WaterSupply

FeedPump

PRV#2

CondenserSuperHeater

Hot WaterSupplyTank

Fan

SteamTurbine

By-PassControlValve

PurgeControlValve


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3.1.1 The Cold Water Supply.

Cold water can be obtained from many sources such as from a municipal water supply, a drilled or dug well, from a river or lake or even from collected rainwater. In any case, the water supplied to any steam turbine should be as clean as possible, free from contaminants and from dissolved minerals. In a closed system, any additional water needed by the turbine is considered to be make-up water, required to replace water lost through PRVs or leaks of one sort or another. Generally, this water can be expected to be cold and will require heat to bring its temperature up close to the boiling point.

Note. If the supply is from a municipal source, from a well, etc., it is likely to be pressurized, which means the entire system will have some pressure at all times.

3.1.2 The Hot Water Supply Tank.

It is suggested that a common 55-gallon household hot water heater can make a suitable hot water storage tank. A typical commercial domestic hot water tank is pressure tested and rated at a pressure of 300 PSI. Typical tanks have a 55-gallon capacity, and have four standard pipe threaded (NPT) penetrations. The two connections located on the top of the tank are for connecting the hot and cold water pipes. The hot water connection at the top of the tank can be connected to provide normal hot water for domestic use and for the feed pump. The upper one, located on the side of the tank, is for a PRV connection, and the lower one is for a drain connection. A one-way valve is built into the cold water intake pipe as illustrated.

In the arrangement shown for a turbine system, the upper PRV penetration requires a tee fitting to connect the output from the condenser to the tank. The PRV that came with the tank must be reconnected to the tee fitting, or replaced with a new PRV.

The lower drain connection can be fitted with a second tee fitting as an alternate connection to the boiler feed pump. The drain valve can then be reconnected to the tee to perform its original function when draining the tank.

[Note for the junk-yard mechanic] It is frequently possible to obtain a discarded water heater tank from a local plumber, because plumbers have to pay someone else to dispose of such tanks. The integrity of the tank may be intact, but the temperature controls may have failed in such a way as to require replacement of the entire unit. Such tanks are usually pressure tested at 300 psi and should be so stamped. The inside of the tank may need to be cleaned before use in the system.



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3.1.3 The Boiler and the Steam Generator.

A typical boiler or steam generator designed to produce steam for driving a turbine consists of two sections. One section includes the portion of the boiler that contains hot water, where the water is turned to steam. The other part heats the steam to a superheated condition. Steam that is not superheated is considered to be saturated steam, or wet steam that contains liquid water. Wet steam is very undesirable for any turbine operation. Steam that does not contain liquid water is considered to be dry steam or superheated steam. All boilers and steam systems are protected from over pressure by safety devices such as pressure relief valves as discussed below.

3.1.3.1 Dangers from a Boiler Failure.

A boiler might consist of a single pressure vessel or container in which water is boiled, or it might consist of an array of straight or bent tubes, or a single tube formed into a coil, or a combination of both. The single coil type is referred to as a mono-tube boiler. The tube type boiler is generally preferred over the pressure vessel because of one very important reason. Whenever a pressure vessel fails, there is inevitably a very destructive explosion due to the sudden release of all the energy contained in the steam. When a boiler tube fails, steam is released, but at a slower rate and less damage can be expected. Nevertheless, any steam that gets loose can be extremely dangerous and life threatening. Anytime a steam leak is suspected, consider it an emergency, shut off the fuel, evacuate the area, blow down the boiler, and wait until the boiler cools before reentering the area.

3.1.3.2 Boiler Water Level.

Any boiler in operation must always have water in it, and must never be allowed to become dry. If a boiler becomes dry and heat is not cut off, the metal may become red hot. If water is introduced to the boiler in this condition, the metal may fail and cause a possible explosion.

One problem with any boiler is to know how much water it contains at all times. The common indicating device is a mechanical, visual glass indicator mounted on the side of a boiler. This problem might be compared to knowing how much fuel is in an automobile gas tank at any time. The fuel tank in an automobile contains a remote sensor and the dash contains a needle gauge or a light and some times an audible indicator. In modern vehicles the remote sensor signal, along with many other signals, are also sent to an on-board computer mounted somewhere under the dash.



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3.1.4 The Super-heater.

For purposes of this discussion the super-heater is considered an integral part of the boiler. In a large power plant the super-heater consists of a separate set of tubes and a header located near the top of the boiler where the temperature within the boiler is the highest. The purpose of the super-heater is to raise the temperature of the steam to the operating temperature required by the turbine to produce a desired output. Because steam in a closed vessel containing liquid water cannot exist at temperatures above the boiling point, the super-heater must be separate from the portion of the boiler containing liquid water. This section must also have the capacity to provide the volume of super-heated steam required by the turbine.

3.1.5 The Boiler Feed Pump.

Where water is heated to steam in a boiler, a pressure exists. The pressure increases as the temperature of the steam increases. As the steam in the boiler is released to turn the turbine, more water must be fed into the boiler to maintain a continuous flow of steam. A certain water level must always be maintained and never allowed to go below a dangerous level. The boiler feed pump forces water from its low-pressure hot water supply tank into the boiler by exerting a pressure greater than the pressure of the steam in the boiler. This pressure however must not exceed the operating point of the pressure relief valve (PRV#1).

In the current example, 647.4 lbs or 78 gallons of water must be forced into the boiler every hour with a pressure not exceeding 150 psia. Therefore the boiler feed pump must be capable of pumping at least 78 gallons per hour at a pressure of 150 psi.

[For the junk-yard mechanic] Check out small pressure pumps and rotary valves and let the author know if you think this is a subject for a future paper.

3.1.6 The Pressure Relief Valve (PRV).

There are at least two places where pressure relief valves are needed. One is at the output or within the boiler section, PRV#1, and the other is at the top of the hot water supply tank, PRV#2.

Note. The boiling point of water increases as the pressure in the boiler increases. This is true in modern automobiles, where the water cooling system is under pressure and the water temperature is above 212 deg F, and where the radiator cap performs the function of a pressure relief valve.



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For information on another type of high pressure valve called a poppet valve, see the following website:

www.answers.com/topic/poppet-valve

The following website shows different types of standard pressure relief valves and offers the reader an idea of the cost for such valves.

www.lowes.com/lowes/lkn?action=howTo&p=Repair/WaterHtrMaintaince.html

The following article offers a discussion of valuable safety features and consideration of steam boiler safety valves.

www.tpub.com/content/construction/14259/css/14259_236.htm

3.1.7 The Turbine.

The turbine is a device that converts one form of energy to another form of rotational mechanical energy. That is a simple, stripped down definition of a turbine. Turbines have been around for centuries. Wind turbines are probably the best known that have been used and relied upon for energy.

The steam turbine of interest in this paper is the bladeless slotted disc turbine. To differentiate it from the Parsons, Tesla, de Laval and other turbines, it can hereafter be referred to as the Saunders Turbine. This is a unique concept among turbines and is the subject of Part 5 of this series of articles.

3.1.7.1 Estimating Turbine Thermal Efficiency.

Note. There have been several accepted methods for determining the efficiency of steam turbines over the years.

The definition of a turbine’s thermal efficiency, as provided by one of the authors of the Katmar Turbine Steam Consumption Calculator, is as follows.

“A turbine is defined as operating at 100% efficiency when it extracts the energy from the minimum quantity of steam possible. Or put the other way around – when the turbine extracts the maximum theoretical quantity of energy per lb of steam. This occurs at the point where entropy of the exit steam is the same as the inlet steam, i.e., there is no increase in entropy. It is termed isentropic operation.

The efficiency of the turbine is the ratio of the actual flow of steam to that which would occur under isentropic (100% efficiency) conditions.”


http://www.tpub.com/content/construction/14259/css/14259_236.htm

http://www.lowes.com/lowes/lkn?action=howTo&p=Repair/WaterHtrMaintaince.html

http://www.answers.com/topic/poppet-valve


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Note. The Katmar Turbine Steam-Consumption Calculator, v2.2 can be downloaded from the following website:

www.katmarsoftware.com/turbine.htm

3.1.7.2 Other Steam Turbine Efficiency Concepts.

The notion of steam turbine efficiency can be difficult to comprehend and to accept. The following bulleted topics are introduced here for the benefit of the reader.

• To appear knowledgeable about steam, always refer to steam conditions, steam properties or steam quality, etc., when discussing steam. Temperature and pressure have little meaning when used alone in describing the characteristics of steam.

• The term efficiency, when referring to steam turbines, is also referred to as the performance value of steam turbines.

• A measure of turbine efficiency is referred to as thermal efficiency, or thermodynamic efficiency.

• A commonly used measure of efficiency is the mechanical energy or shaft energy out divided by the net energy-in, e.g., kWh per Btu.

• The water rate: lbs of steam required to generate a kilowatt-hour or a horsepower-hour of energy … is not very useful when used alone to describe turbine performance.

• Rankin-cycle efficiency: available heat per lb / total heat input per lb• The thermal efficiency: percentage of the total heat input of the steam consumed by

the turbine which is converted into work.

Note. For a somewhat exhausting discussion of thermal efficiency, see the following website:

en.wikipedia.org/wiki/Thermal_efficiency

3.1.8 The Condenser. The function of a condenser is to remove excess heat from the exhaust steam and to convert the unused steam back to a liquid state. If the turbine is 25% efficient, then it might seem like a pure waste to throw away or lose 75% of the energy. However, the efficiency in a CHP system can be very high where the exhaust heat is fully utilized.

Commercial power plants that generate electricity used by consumers face the same problem of getting rid of exhaust heat. Some of it is used internally in places like pre-heaters and re-heaters. However, most large power plant facilities are located along the shores of oceans, lakes, and rivers. One purpose for this is to be able to transport fuel in barges. Another advantage is to have a place to dump excess heat from the condensers.

[Note for the junk-yard mechanic] One or two truck or automobile radiators with fans from a local auto junk yard may provide an inexpensive condenser.



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The following website offers a detailed explanation for condensers typically used in power plants. The author introduces the reader to the term Latent Heat of Condensation.

www.engineersedge.com/heat_exchanger/large_steam_condenser.htm

Note. The reader is encouraged to search the internet for more discussions about the system components discussed in this paper. However, it is the intent of this author to keep things simple wherever possible.

3.1.8.1 Using the Exhaust Heat.

Most of the exhaust heat can be saved if it can be used to heat a living space, a swimming pool or for some similarly important purpose. In such cases the system is known as a cogeneration system, or in some circles a combined heat and power, CHP, system . Such an example is found in the heating system in an automobile.

The radiator in an automobile performs a function similar to the condenser in a steam system. Excess heat from the engine block is carried to the radiator by circulating water and in this case one objective is to remove as much heat as necessary to prevent the water from boiling.

[Note for the junk-yard mechanic]. It is possible to use a discarded automobile or truck radiator with a working fan from a junk yard for the condenser in a small steam turbine system.

3.1.9 The Steam By-Pass Valve.

The steam by-pass valve is a control device rather than a safety device, and can be either a solenoid operated valve or a proportional valve. The purpose of this valve is to provide a means for regulating the flow of steam through the turbine when relatively small adjustments are needed in the output of the turbine. The by-pass valve, when used, routes steam around, or bypasses, the turbine. This device can safely be omitted in a small solar system.

3.1.9.1 The Water By-Pass Valve.

A water by-pass valve can be used to manually control the amount of water in the boiler as described for the Stanley Steamer in the following interesting article:

www.stanleymotorcarriage.com/Parts/WaterAutoBypassValve.htm


http://www.stanleymotorcarriage.com/Parts/WaterAutoBypassValve.htm

http://www.engineersedge.com/heat_exchanger/large_steam_condenser.htm


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3.1.10 The Purge Control Valve.

For purposes of this discussion, the Purge Control Valve or Blow-down Valve is a safety device, but it is also used to purge the steam lines of water and moisture at start-up or shut-down times. It may also be used as a maintenance device as discussed in the following website:

www.mckenziecorp.com/boiler_tip.htm

3.1.11 The Alternator and Generator Head.Here are some basic things to consider:

• Engines produce high torque and run most efficiently at relatively low speeds.• Generators require relatively high torque and run at lower speed.• Turbines produce relatively low torque and run most efficiently at high speeds.• Alternators require relatively low torque and run at higher speed.

In a rotating body, energy is proportional to the product of speed and torque. Therefore, speed and torque can be traded off by including a gearbox between the motor and alternator, or between the turbine and generator. In an automobile a system of belt pulleys is used. The pulley ratio to the alternator is such as to keep the alternator operating at idle engine speeds.

The following article compares the operation of generators and alternators for use with wind power. The concepts and principles are also valid for steam turbines.

www.otherpower.com/otherpower_wind_alternators.html

The following articles are similar to the above except there are more detailed diagrams for alternator circuit hookups for charging batteries.

www.alternatorparts.com/understanding_alternators.htm

eduhosting.org/windpics/altcomp.html

Where ever this paper is focused on the application of solar energy using small turbines in the 1 to 3 kW range, the use of alternators is assumed.

[Note for the junk-yard mechanic] Rebuilt alternators can be purchased at local automotive parts stores and used alternators can be found at local auto junk yards. It is recommended that the minimum size alternator needed for a 10kW system should be rated at 100 amps continuous and should have a built-in regulator.


http://www.alternatorparts.com/understanding_alternators.htm

http://www.otherpower.com/otherpower_wind_alternators.html

http://www.mckenziecorp.com/boiler_tip.htm


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3.1.12 The Controls.

The operation of any turbine system must be safe and requires some automatic control devices that enable an orderly shut-down in a timely manner in case of emergency or other malfunction. The heart of any large system is usually a controller with an integrated computer processor.

3.1.13 Controls and Safety Devices for Small Systems.

The only control devices in the small system described above are two pressure relief valves, PRV #1 and PRV #2, the by-pass valve and the purge control valve. The pressure relief valves are mechanical and operate automatically. The purge control valve could be manually operated but should be a remotely operated solenoid controlled device.

Although the need for safety devices is considered minimal, they are considered absolutely essential. In small systems the lower operating pressures and temperatures are not hazardous when reasonable safety precautions are followed. One of the greatest hazards in any turbine system occurs when there is a loss of the load resulting in a runaway turbine. A means must be included in the system to detect such a condition and to automatically operate the purge control valve or blow-down valve as an emergency shut-off procedure.

home.howstuffworks.com/home-thermostat1.htm

Note. Any reader interested in becoming certified in one or more areas of steam turbine technology is encouraged to review the following website:

www.hpcnet.com/schedule/topic.html#tg401


http://www.hpcnet.com/schedule/topic.html#tg401


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4 An Example: Making Steam for 10 kW.

As previously stated, the subject of steam properties is complicated, confusing and tedious, especially if it is necessary to use published steam tables.

Using the Turbine Steam Consumption Calculator, Version 2.1 from Part 2, enter the following:

Input data:Inlet Steam Press (abs): 165 psiaInlet Steam Temperature: 600 FahrenheitExhaust Pressure (abs): 15 psiaTurbine Efficiency: 25 PercentTurbine Power: 10 kW

Inlet steam properties:

Saturation Temperature: 365.9 FahrenheitEnthalpy: 1324.6 Btu/lbEntropy: 1.7 Btu/lb-deg F

Exhaust steam properties:

Enthalpy: 1271.9 Btu/lbEntropy: 1.908 Btu/lb-deg FTemperature: 467.9 Fahrenheit Degree superheated 254.7 Fahrenheit

Steam consumption:

Specific: 64.72 lb/kWhActual: 10.79 lb/min

For the above inlet conditions and a thermal efficiency of 25%, the difference between the Inlet Steam Enthalpy and the Exhaust Steam Enthalpy is as follows:

H = H1 – H2 = 1324.6 Btu/lb – 1271.9 Btu/lb = 52.7 Btu/lb

This is the maximum possible amount of energy that can be extracted per lb of steam by the turbine.



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Note. psia stands for pounds per square inch absolute. To find the inlet gauge pressure, psig, subtract the absolute pressure, psia, at the exhaust port of the turbine from the absolute inlet pressure, psia. Use atmospheric pressure of 14.7 psia (or approximately15 psia) as a base line. See the following for a definition of psig

en.wikipedia.org/wiki/Pounds_per_square_inch

4.1.1 Calculate Water Rate and Heat Needs Using 25% Thermal Efficiency.

Calculate the amount of water in gal/hr that needs to be converted to superheated steam:

W’ = 10.79 lb/min x 60 min/hr = 647.4 lb/hr

W = 647.4 lb/hr / 8.3 lb/gal = 78gal/hr

Note. In Figure 2, the typical hot water supply tank might have a capacity of 55 gallons. In a closed system, more than 55 gallons per hour can be produced by re-circulating the water from the condenser. It can be assumed for this example that the temperature of the water from the condenser, and thus the water in the hot-water supply tank, is near the boiling point.

How much heat is required to convert 647.4 lb. of water at 212 deg F to steam?

From Part 1: Latent Heat of Vaporization for water = 970 Btu / lb

Therefore the estimated heat required to convert the amount of water needed to produce the desired amount of steam is:

Hv = 970 Btu / lb x 647.4 lb/hr = 627,978 Btu/hr

How much more heat is needed to raise the temperature of the steam to 600 deg F?

From Part 1 it was estimated that the heat required to increase the temperature of steam is 0.5 Btu / lb/ deg F of steam. This estimate is oversimplified but allows the discussion to continue. Therefore the heat needed to raise the steam temperature to 600 deg F is:

Hs = 647.4 lb/hr x 0.5 [Btu / lb-deg F] x (600 – 212) deg F = 125,596 Btu /hr

The total heat needed to raise the boiling water to 600 deg. F superheated steam is:

Ht = Hv + Hs

Ht = 627,978 Btu/hr + 125,596 Btu /hr = 753,574 Btu / hr



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4.1.1.1 Physical Size Estimate for a 10 kW Solar Collector.

Now that an estimate of heat needed to run a 10 kW turbine is known, the next step is to estimate the size of the solar collector required to produce that amount of energy.

If at some point and time the solar power density is 1.0 kW/m^2, then there are 3412 Btu/m^2/hr available from the sun. Therefore the area of the collector required for this example is:

A= Area of the collector.

A = 753,573 (Btu / hr) / 3412 (Btu / hr / m^2) = 221 m^2

Obviously, a 3-meter parabolic satellite dish with an area of approximately 7 square meters is not adequate in this case. A trough collector described in Part 4 of this series can be designed and built to satisfy this requirement.

4.1.1.2 Exhaust Steam Conditions.

The exhaust steam must remain dry and superheated and cannot be allowed to reach a saturated condition. That means that the 970 Btu/lb of latent heat of evaporation is notused during the operation of the turbine, but is removed by the condenser. Unless this heat can be used in a co-generation or CHP environment, it can be considered wasted energy.



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4.1.2 Calculate Water Rate and Heat Needs Using 100% Thermal Efficiency.The reader is encouraged to make the same calculations for 100% thermal efficiency using the calculator, version 2.2. Version 2.2 of the calculator reports Exhaust Steam is Wet if the exhaust steam is not dry, where version 2.1 reported Degrees Superheated. (hit the OK button to continue)

Input data:Inlet Steam Press (abs): 165 psiaInlet Steam Temperature: 600 FahrenheitExhaust Pressure (abs): 15 psiaTurbine Efficiency: 100 PercentTurbine Power: 10 kW

Inlet steam properties:

Saturation Temperature: 365.9 FahrenheitEnthalpy: 1324.6 Btu/lbEntropy: 1.7 Btu/lb-deg F

Exhaust steam properties:

Enthalpy: 1113.7 Btu/lbEntropy: 1.7 Btu/lb-deg F NB Same as InletTemperature: 213.2 Fahrenheit Quality 0.9617 1.0 = sat. vap.

Steam consumption:

Specific: 16.18 lb/kWhActual: 2.697 lb/min

For the above inlet conditions and a thermal efficiency of 100%, the difference between the Inlet Steam Enthalpy and the Exhaust Steam Enthalpy is as follows:

H = H1 – H2 = 1324.6 Btu/lb – 1113.7 Btu/lb = 210.9 Btu/lb

Note the Inlet Entropy and Exhaust Entropy are equal. This is state for the maximum possible amount of energy that can be extracted per lb of steam by the turbine. The percentage of the maximum possible energy actually being extracted per lb of steam is:

Efficiency = 52.7 (Btu/lb) / 210.9 (Btu/lb) = 0.25 = 25%



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5 Summary of Part 3.

Note. This analysis is from one of the authors of the Katmar calculator:

“An important point to note, and what confuses many people, is that this efficiency is the thermodynamic efficiency. It is very different from the efficiency in terms of what energy in the steam finally ends up as useful work in a shaft.

… we see that of the 1271.9 Btu/lb that was in the steam, , only 210.9 Btu/lb is actually made available as shaft work, i.e., only 16.6% of the energy that was in the steam ends up as shaft work.”

It is completely acceptable that the turbine in an experimental system is not expected to operate at 100% thermal efficiency, but rather at some lower level, e.g., 25%. Such a system is inherently simple, without sophisticated instrumentation to control its operation based on efficiency. It is the overall ability to produce electricity from the sun that is important. Efficiency becomes less of an issue when the fuel is free.

5.1 Preview of Part 4.

It has been shown that a 3-meter parabolic dish can be useful for making hot water. Except as otherwise noted, it is probably not very useful for making enough steam to make electricity by driving a turbine or other steam engine. Part 4 concentrates on the analysis and design of trough collectors that can produce enough steam to drive a 10 kW generator.

5.2 Notes to the reader:

1. Part 4 will focus on trough type solar collectors.2. In Part 5 the structure and properties of the slotted disc turbine will be discussed.3. In future articles, additional discussion of higher-powered systems using other fuels

will include the use of generators with turbines in the 5 kW to 10 kW ranges.4. Comments from the reader regarding the material covered are welcome.5. Any errors or omissions reported to the author would be appreciated.


solar white-paper part3 10

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