Solar Thermal Cogeneration
[Overview] [Program] [System] [Collector] [Turbine] [Generator] [Controller] [Battery]



By drawing our energy directly from the sun we minimize geological, ecological and social disturbance.  The Solar-Thermal Cogeneration (STC) System is a complete energy system, providing both electrical and thermal energy for a detached residental or commerical structure.  Solar thermal describes converting the sun's radiation into usable heat energy and cogeneration describes generating electricity while using excess heat for thermal needs such as space heating/cooling.  Cogeneration is sometimes called combined-heat/power.  High efficiencies can be achieved with cogeneration, making the STC competitive when full costs and benefits are considered.  A drawback of cogeneration is less flexibility in independently specifying the electric and thermal capacities.


The STC system provides residential energy in a flat-roof-mount configuration.  The system can supply all electric and thermal needs for a conservation-oriented residence.  On a clear winter day in a sunny climate in mid-latitude an STC system occupying 1000 sq. ft. of roof area can provide 25 kilowatt-hours of electric energy and another 56 kilowatt-hours of thermal energy for space heating, water heating and refrigeration.  This provides 16 to 19 kilowatt-hours of residential electric energy plus 15 to 30 miles of range per day for an electric vehicle.  In summer these outputs are multiplied by 2.5, providing an extra 125 miles of range per day for electric vehicles, and plenty of thermal energy for space cooling.  According to the J.T. Lyle Center *, for a given latitude, the least sunny climates yield at least 2/3 of the capacity of the sunniest climates, indicating widespread feasibility for solar-based systems.

The STC system employs a Rankine cycle heat engine with a closed-loop water-steam boiler.  Components include a solar collector, turbine/generator, and electrical and thermal output subsystems. The collector consists of a concentrator (optical reflector) and a receiver (heat pipe).  The reflector geometry is parabolic trough, a practical geometry for low-profile roof mounting.  A tracking mechanism maintains concentrator focus as the sun traverses the sky.  The turbine is of the boundary-layer type and drives a permanent magnet generator.  The electrical output subsystem includes lead-acid batteries, high-efficiency charge controller and AC /DC distribution controllers.  The thermal output subsystem includes ammonia/water absorption space cooling/refrigeration, and space heating/water heating.  The steam circuit is operated with water at night to radiate the system's waste heat to the sky and to supplement space cooling.

The STC program provides extensive design information, fabrication procedures, and supplemental resources, enabling widespread production of STC systems in small workshops with simple tools, limited skills, and available materials.

Roof Mount for Economy and Aesthetics

views of house with STC mounted on roofThe STC is intended for mounting on a flat roof.   A roof parapet may completely or partially hide the concentrator.  The concentrator might partly show above the parapet during the winter to reach more sun in the low sky but hide below the parapet during the summer.  The equator-facing wall might have a lower parapet for better exposure.  Besides aesthetics, roof mounting preserves wildlife habitats, economizes space, economizes energy transfer, and keeps sunlight off much of the roof. 

Comparison With Alternative Approaches

Photovoltaic and wind systems are two alternative approaches to generating electric power.  Photovoltaic has no moving parts and requires less maintenance than solar-thermal systems.  Photovoltaic panels are becoming less expensive but their manufacture is enormously resource-intensive (electricity, water) and capital-intensive, with associated environmental and social costs.  Schott has calculated that photovoltaics take five years to produce the energy embodied in their manufacture while solar-thermal and wind systems take five months. 

In 2005, high labor costs kept photovoltaic costs high and material shortages were expected to persist for years. Photovoltaic panels lose as much as 90% of energy absorbed to heat.  This heat may be collected and utilized but it is not as versatile (i.e. hot) as that produced by concentrated solar-thermal.  Photovoltaic panels are also expensive to recycle. 

Wind systems may be more economical than solar-thermal systems, having fewer mechanical/thermal components, but they can be lethal to wildlife and an aural and visual blight for some people.  In a six-week period, FPL's 44-turbine wind facility in West Virginia killed an estimated 1,364 to 1,980 bats [1].  Complaints are fairly frequent and zoning restrictions and permit fees are likely in suburban/urban communities [2].  Wind energy is more erratic and off-peak than solar, often requiring a larger battery reserve. 

Photovoltaic and wind systems may generate electricity in tandem with a low-temperature solar thermal system for hot water and space heat.  Refrigeration and space cooling require doubling to tripling the electrical capacity, including batteries, or fuel heat.  Solar-thermal systems economize by utilizing the waste heat from electric generation to supply all thermal loads.  Improvements will continue to be made to both photovoltaic and wind technologies but solar-thermal is a complete energy system with low full costs and is especially good for a flat-roof structure in a sunny climate.

Availability of either wind or solar energy at a given location should be researched early.  Wind and/or solar measurements should be collected over a period of one year, or preferably more, in the immediate vicinity of the installation.  Buildings and trees may interfere with both wind and solar systems, probably more with wind.

[1] Yellow light for a 'green' energy source |
[2] A War Over Wind Turbines - Los Angeles Times - December 7, 2005

Residential Energy Needs

Residential energy has been conventionally supplied by grid electricity and piped natural gas and fuel oil.  The STC may supplement these sources for flexibility, or entirely replace them, which may be simpler and in some cases more feasible.  When replacing them, it is important to size the STC carefully.

Estimating residential energy needs is an art, not a science.  Energy needs can change, for example, with the number of residents.  A strategy to address this dynamic, and reduce investment risk, is to start with a minimum system and expand as needed.  The STC's solar collector, thermal banks and batteries are expandable.  The rest of the system must be sized initially to accommodate the later expansion.

While the the energy from the sun is free, energy systems still have embedded costs.  To minimize the needed system capacity for active heating/cooling, ideally, new construction would employ passive solar/passive design techniques.  Passive design can completely satisfy space heating/cooling requirements in many climates.   If an electric vehicle is planned that would likely become the primary energy consumer resulting in a surplus of thermal energy. 

The table below shows the US DOE's data of average residential energy consumption in four large US states.  Space heating and cooling represents a minority percent of total energy usage in warmer climates.  The remaining applications include water heaters, refrigerators, stoves/ovens, lights, clothes and dish washers, televisions and computers. 

1997 residential
(million BTUs/year)

space heating

space cooling


31 12 53 96
New York
77 2 44 123
20 4 40 64

In a conservation-oriented household powered by the STC, water heating and refrigeration are handled as thermal loads.  Clothes and dish washing and drying can be handled the same (the STC program will later expand to include appliance designs).  Cooking duty might be shared between a biofuel stove and a solar oven.  Low-power appliances utilize efficient motors and avoid large transformers and illuminated displays.  Low-power televisions and computers are available with LED-backlit LCD displays.  Generally, laptop computers are lower power than desktop models. 

Small-Scale Distributed Energy Systems

Small-scale distributed systems enable the efficiency of cogeneration but have additional advantages over large-scale systems.   Small-scale distributed systems have reduced development, security, environmental, and transmission costs.  Small-scale systems can preserve the landscape if held to certain constrains, e.g. solar-thermal systems mounted in low-profile configurations on the roofs of building structures.  The materials cost of small-scale energy systems beyond that of large-scale production/transmission is very small.  In particular, small scale solar concentrators require less structural strength against wind forces than larger scale concentrators.  Small-scale systems protect consumers from market turmoil that may arise from speculation, geopolitics, and natural disasters. 

In turbine/generators, the material's molecular bonding strength is the same in small and large scale equipment, so the large scale equipment has considerably more mechanical stress/fatigue problems.  Large-scale turbine/generators must be taken offline and rebuilt as often as every six months.  A small-scale turbine/generator may last a decade before it needs rebuilding.  The engineering cost of small scale systems is much lower.

One of the environmental costs of centralized energy production is the condensing stage of the Rankine cycle, which is used in the vast majority of electric generating plants.  This process alters ecosystems, by evaporating and/or warming the water that is the habitat of many species.   In industrialized nations the volume of water affected is nearly equal to the total amount otherwise consumed by the human population.  In 2006 it was reported that Global warming is inducing water shortage, forcing centralized electric plants to scale back production (see Global Warming).  In 2007, the US EPA was seeking to classify water vapor expelled into the atmosphere by human activities as a pollutant *.

Environmental effects of the waste heat from the distributed heating/cooling of residential and commercial structures are not well known at this point but one of the STC design goals is to emit as much of the waste heat as possible to the sky.

Industrial and Commercial Cogeneration (pdf) - US Office of Technology Assessment - 1983 (archive):

Congress has expressed considerable interest in decentralized energy systems and in cogeneration. Cogeneration is a major issue in the National Energy Act of 1978, parts of which were designed to remove existing regulatory and institutional obstacles to cogeneration and to provide economic incentives for its implementation. In addition, the House Energy and Commerce Committee, the House Science and Technology Committee, and the Senate Energy and Natural Resources Committee have held hearings on cogeneration, and several committees in both Houses of Congress have held hearings on the general concept of decentralized energy systems.

Merging new power with old system proves costly
Microgeneration Technology: Shaping Energy Markets

Local Craftsman Industry

The STC is designed to be built in local workshops from as raw materials as possible.  Things that can be built and maintained locally bring economic opportunity to the local level.  The knowledge, skills and tools acquired to build STCs can be used to build other things.

The STC design is fully documented online. High quality documentation helps enable better construction and maintenance of the systems, extending system life and reducing costs.  Undocumented designs can result in poor maintenance and shortened system life.  The STC documentation helps reasonably skilled craftsmen successfully construct and maintain the systems and make successful material substitutions and design alterations.

Commercial components may be purchased and connected together to make a solar-thermal cogeneration system.  These components should be high-quality, well-documented and serviceable to meet the target service life.  The STC design may include some commercial components which meet these criteria.

Photovoltaic Material Shortage
Silicon Still in Short Supply - June 21, 2005 - The Energy Blog
The shortage on Solar Grade Silicon was evident at the 20th European Photovoltaic Solar Energy Conference, Barcelona, Spain and Exhibition last week. With the resulting shortages in wafers, solar cells and modules, it was a major topic and it seems it will remain high on the agenda for at least the coming 2 years.

Shortages Stifle a Boom Time for the Solar Industry - August 6, 2005 - New York Times
American suppliers for the solar energy industry say that burgeoning demand both domestically and overseas, a weak dollar and shortages of raw material have created back orders of several months on electricity-generating photovoltaic, or PV, panels.

Due to a world-wide shortage of Photovoltaic Panels we are unable to offer specific models for purchase here at this time, however, with our connections to suppliers and manufacturers, we are able to find panels to meet your needs.

Global Warming

Global warming is a trend of increased atmospheric and terrestrial temperature due mainly to the greenhouse effect from carbon dioxide from combustion of fossil fuels and livestock, and increasing feedback effects such as the albedo change in the arctic with ice melt, methane release from thawing Siberian peat bogs, etc.  The increased temperature leads to erratic weather patterns, rising sea levels causing coastal flooding, and adverse pressures on societies, ecosystems and biodiversity, i.e. species extinctions.  Additional pollutants from combustion of fossil fuels include sulfur oxides, nitrous oxides, carbon monoxide, ozone, particulates and mercury.

glacier 1932 glacier 1988
(sierra club)

Greenhouse Gas






Calif. 1999 (%)

USA 1998 (%) 32
USA 1997 (*)

USA gas (*) 35

USA oil (*) 20

USA coal (*)

*millions of metric tons of carbon equivalent

Nigeria kidnappers demand $1.5bn and control of oil, Times Online, 1-17-2006

Hundreds burned alive in Lagos pipeline blaze - The Guardian UK, 12-27-2006

More than 260 people were killed yesterday - burned alive when a ruptured oil pipeline burst into flames in Lagos. Crowds of local residents had gone to scoop up petrol using plastic containers after an armed gang punctured an underground pipeline to illegally siphon off fuel.  The Christmas shortage has caused all the more hardship as petrol is not only essential for transport, but for the generators on which most homes and businesses rely in the absence of reliable mains electricity.

Global Warming, Not Just Heat Wave, Inter Press Service, 7-21-06

Gerstengarbe said that over the last century temperatures in Germany rose 0.8 degrees. "Over the next 75 years, we expect a warming of between 1.8 to 3..6 degrees for our region."  The heat is also taking its toll on agriculture, and affecting the generation of electricity, especially in nuclear power plants.  The lack of fresh water for the nuclear plants' cooling systems has led German private electricity suppliers to slow down their generators.  In France, the state-owned Electricité de France (EdF) was allowed to continue to drain hot water from the cooling system into rivers, although the water temperatures exceeded the limits imposed by environmental authorities. But output has had to be lowered.  EdF has been importing electricity to compensate the nuclear power plants' lower performance. In Italy, hydroelectric plants have had to slow down due to a shortage of water in rivers.


Report: Future Bright for Solar Thermal - Green Inc. Blog
Key Oil Figures Were Distorted by US Pressure - IEA projections shown to be unreliable

NRECA's Distributed Generation & Toolkit for consumers
Microgrids as peer-to-peer energy -
IPCC - Intergovernmental Panel on Climate Change
Dirty Air/ Dirty Power (pdf) : Coal adds $167 billion to annual US healthcare bill


Copyright (c) 2005-2009 Robert Drury
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