Solar Thermal: Pros and Cons – Part 1: Solar Heating and Cooling

There is no perfect energy source. Each and every one has its own advantages and compromises. This series will explore the pros and cons of various energy sources.  Learn about other forms of energy generation here.

The term solar thermal has been used to describe two different types of systems. One is where solar panels are used to collect heat, which is used directly, as domestic or process hot water, space heating, or in some cases, air conditioning. This is really the most basic form of solar energy utilization, which is commonly known as solar heating and cooling (SHC). Technically, one could consider drying ones clothes on a clothesline at one end of the solar thermal energy spectrum, along with the passive solar energy that comes in through the window to warm your house on a cool spring day.

The other, very different type of system involves concentrating solar collectors focusing an intense beam on a vessel or pipe containing fluid, which is then converted to steam to drive an otherwise conventional thermal power plant. This is generally called concentrated solar power or CSP. We will cover this latter type in a separate post under the heading Concentrated Solar Power.

As for solar heating and cooling, according to the International Energy Agency (IEA), this sector grew by 14 percent in 2010. A total of 162 billion kWh of heat was collected, making this second only to wind, of the new, clean tech renewables.

The vast majority of installed systems were in China and Europe which combined to account for 78.5 percent of the total capacity.

Most of these systems, 95 percent, were for domestic hot water only, primarily utilizing gravity flow, or thermo-siphoning. Only 11 percent required active pumping, making this technology ideally suited for installations both with and without electricity. All together, some 10.2 million square feet of thermal collectors were installed in 2009. There are a wide variety of systems available, ranging from sophisticated systems with vacuum tubes that interface with radiant floor heating, to simple home-brew systems that can be little more than pipes mounted on a black painted board.

For some reason these types of systems have not really caught on in the US. EIA shows sales more than doubling from 2000-2006, but declining since that time.

The EIA divides solar thermal into three categories, low temperature (primarily for swimming pool heaters), medium temperature (for domestic hot water), and high temperature (systems used by utilities to generate electricity. Most of the systems shipped in the US, some 73 percent, were for swimming pool heaters. In this article we are only considering low and medium temperature systems. These are sometimes referred to as 1x sun systems, meaning that the sunlight is not significantly amplified or concentrated.

I’m not sure why more people here don’t heat their hot water with solar. These systems can be quite inexpensive and simple and can pay for themselves fairly quickly, but somehow they seem to lack the luster of the far more expensive photovoltaic systems which are only one-third as efficient in delivering electricity than these systems are in delivering heat. Activity might be picking up though. One 138 unit apartment complex in Tucson just opted for solar hot water instead of PV. They worked with the manufacturer Free Hot Water in San Jose.  The system is expected to save $1200 per month and roughly 80,000 pounds of CO2 per year.

Solar cooling, though it sounds like an oxymoron, is actually quite real and can be very practical. It is based on the absorption cycle, the same principle that drove some of the earliest refrigerators and is still used in the propane refrigerators that are found in many RVs and trailers. Essentially, they use heat to compress the refrigerant in a fixed volume instead of a mechanical compressor. The rest of the cycle is basically the same; the cooling is produced as the compressed liquid as is expanded in the evaporator. (Think of how cold spray deodorant feels coming out of an aerosol can.) The great thing about solar cooling is that the energy is always available when you need it the most, when the sun is out. Most of these systems today are still complex and expensive so that they only make sense at a commercial scale.

For the most part, it’s easy to separate solar thermal systems and solar photovoltaic systems into two categories. But, there are new hybrid systems that combine both for cogeneration and even tri-generation, which produce electricity, heating and cooling. These systems are inherently more efficient than either system alone, since more useful energy is being extracted from the incoming sunlight. We covered one example of a company doing this, Naked Energy, last month.

Solar thermal: Heating and Cooling


  • Renewable. No fuels required.
  • Non-polluting. Carbon free except for production and transportation
  • Inherently distributed with onsite production
  • Simple, low maintenance
  • Solar cooling is available when you need it most
  • Hot water provide some limited storage capacity
  • Operating costs are near-zero
  • Quiet. Few or no moving parts.
  • Mature technology
  • Good ROI
  • High efficiency
  • Modular systems
  • Can be combined with photovoltaics in highly efficient cogeneration schemes.


  • Intermittent
  • Low energy density
  • Does not produce electricity
  • Supplemental energy source or storage required for long sunless stretches
  • Expensive compared to conventional water heaters
  • Construction/installation costs can be high
  • Harder to compete against very cheap natural gas
  • Some people find them visually unattractive
  • Manufacturing processes can create pollution
  • Installers not available everywhere
  • Generally not practical to store or sell excess heat
  • Produce low grade energy (heat vs. electricity)
  • Limited scalability
  • Dependent on home location and orientation

These systems tend to be small scale and are certainly not the silver bullet to solve our energy challenge. After all, domestic water heating, comprises only 22 percent of residential energy consumption. On the other hand, these systems are available and affordable and if many more people started using them, it would make a huge difference, as one aspect of a multifaceted solution.


What about other energy sources?

[Image credit: Seattle.roamer: Flickr Creative Commons]

RP Siegel, PE, is the President of Rain Mountain LLC. He is also the co-author of the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water in an exciting and entertaining format. Now available on Kindle.

RP Siegel

RP Siegel, author and inventor, shines a powerful light on numerous environmental and technological topics. His publications include business and technical articles as well as books. His third, co-authored with Roger Saillant, is Vapor Trails, an adventure novel about sustainability. RP is a professional engineer - a prolific inventor with 52 patents and President of Rain Mountain LLC a an independent product development group. He is also active in his community of Rochester, NY. A regular contributor to Mechanical Engineering magazine, RP recently returned from Abu Dhabi where he traveled as the winner to the 2015 Sustainability Week blogging competition.. Follow @RPSiegel on Twitter. Contact: