Renewable thermal energy

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Renewable Thermal
Type Energy
Working principle Thermodynamics
First production 1800's

Renewable thermal energy is the technology of gathering thermal energy from a renewable energy source for immediate use or for storage in a thermal battery for later use. An example of Renewable Thermal is a Geothermal Heat Pump (GHP) system, where excess thermal energy due to solar heating from the sun is removed from the structure via the heating and cooling system and stored in the ground, and that same energy is then extracted from the ground to later heat the same building in another season. This example system is "renewable" because the source of excess heat energy is a reliably recurring process that occurs each summer season; in this case it is even a natural renewable energy source.

History of Renewable Thermal Systems[edit]

The outer crust of the Earth is a Thermal Battery that maintains a median temperature which is the same as the average air temperature at that location. This "average ground temperature" is a combination in balance of solar gain from the sun, thermal gain from the core of the earth, and heat loss due to conduction, evaporation, and radiation. The graphic at the right shows a map of the "average ground temperature" at locations within the United States.[1]

Solar-based Renewable Thermal[edit]

A ground heat exchanger (GHEX) is an area of the earth that is used as an annual cycle thermal battery. These thermal batteries are un-encapsulated areas of the earth into which pipes have been placed in order to transfer thermal energy. Energy is added to the GHEX by running a higher temperature fluid through the pipes and thus raising the temperature of the local earth. Energy can also be taken from the GHEX by running a lower temperature fluid through those same pipes.

GHEX thermal batteries are implemented in two forms. The picture above depicts what is known as a "horizontal" GHEX where trenching is used to place an amount of pipe in a closed loop in the ground. GHEX's are also formed by drilling boreholes into the ground, either vertically or horizontally, and then the pipes are inserted in the form of a closed-loop with a "u-bend" fitting on the far end of the loop. These drilled GHEX thermal batteries are also sometimes called "borehole thermal energy storage systems".

Heat energy can be added to or removed from a GHEX Thermal Battery at any point in time. However, they are most often used as an "Annual-Cycle Thermal Battery" where energy is extracted from a building during the summer season to cool a building and added to the GHEX, and then that same energy is later extracted from the GHEX in the winter season to heat the building. This annual cycle of energy addition and subtraction is highly predictable based on energy modeling of the building served. A Thermal Battery used in this mode is a Renewable Energy source as the energy extracted in the winter will be restored to the GHEX the next summer in a continually repeating cycle. This Annual-Cycle Thermal Battery is a solar powered thermal storage because it is the heat from the sun in the summer that is removed from a building and stored in the ground for use in the next winter season for heating.

Process-based Renewable Thermal[edit]

There are many processes that have been created by mankind that produce thermal energy on a very reliable and repeatable basis. One giant example of such a reliable producer of thermal energy is a utility electric generator. The very process of generating electricity in a generator produces more heat than it does electricity, and this heat energy can be captured and used. When such Byproduct Thermal Energy is use immediately, this is commonly called Cogeneration. However, if heat that would have been wasted because it is not immediately needed is instead captured and stored for use at a later time, this can be an example of Renewable Thermal. The key aspect of Renewable Thermal is that it involves storing and later using energy which is reliably then available for capturing and storing again.

Renewable Thermal In The News[edit]

The state of New York took a big step in September 2015 when it created a new office titled Director of Renewable Thermal.[2] The NY Director of Renewable Thermal will oversee a team to help companies develop and implement renewable, low-carbon cooling and heating systems. NY State considers this initiative a critical component of NYSERDA’s strategy to enable net-zero energy buildings, which produce the same amount of energy as they consume. It also will further advance New York’s progress toward creating self-sustaining energy markets for clean, renewable technologies.

Renewable Thermal has been a core resource in many states Renewable Portfolio Standards.[3] The report says: "State Renewable Portfolio Standard (RPS) programs have historically focused on electricity generation. However, some states have started incorporating renewable thermal power for heat generation into their RPS as a way to support the development and market growth of solar thermal, biomass thermal, geothermal, and other renewable thermal technologies." Further: "Renewable thermal energy has many of the same benefits as other renewable technologies, including improved air quality, economic development and job creation, and the promotion of regional energy security."

Importance of Renewable Thermal[edit]

In a recent article,[4] Bill Nowak, the Executive Director of the NY-GEO industry trade group, stated: "According to the recently adopted New York State energy plan, on-site combustion (largely for heating buildings) is responsible for 35 percent of fossil fuel greenhouse gas emissions in New York State. In-state electricity generation is responsible for only 18 percent. We strongly support cleaning up electricity generation in New York, but stress that renewable thermal is the next wave in resisting climate change."

See also[edit]


  1. ^ [1], US Average Ground Temperature Map
  2. ^ [2], NYSERDA Announces Donovan Gordon to Lead Effort to Expand Renewable Cooling and Heating Markets in New York, September 16, 2015
  3. ^ [3], Renewable Thermal in State Renewable Portfolio Standards, April 2015
  4. ^ [4], New Bills May Be Game Changer for New York Geothermal, September 15, 2015