Renewable thermal energy

Solar energy has been in use for centuries for heating dwellings and to produce hot water before low cost natural gas was discovered. It gained attention during and after the oil embargo of 1973 as engineers investigated ways to produce thermal energy from a renewable source instead of fossil fuels. The history of utilizing the ground as a heat source is more recent and has gained prominence in recent years especially in rural areas where natural gas heating may not be available. 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 energy is considered to be the most popular form of renewable thermal energy in the world. Solar thermal energy is generally collected by either liquid or air solar collectors. Liquid solar collectors are used to heat water for domestic hot water, process applications and for swimming pools. Millions of solar water systems are being used world wide. Air solar collectors are normally used to heat buildings and for processes such as crop drying. Air collectors are typically building integrated on south facing walls to maximize the low winter sun angles when buildings in cold climates require heating.

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.

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."

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."

• Thermal energy storage
• Seasonal thermal energy storage system
• Solar air heat
• Geothermal heat pump