Solar Engineering Applications

Energy Use, Energy Storage

1. A QUICK LOOK AT HOW WE USE ENERGY

Human societies derive the energy they use from primary energy resources; that is , from energy resources that exist in nature, unaltered by man. A list of commonly used primary energy resources would include coal, oil, natural gas, wood, the gravitational potential energy of water, wind, sunlight, etc. Primary resources can be either renewable or non-renewable. Regardless of this distinction, these resources are not usually in a form that is directly useful. An important function performed in all societies is the transformation of energy available from primary resources into the secondary energy forms that we actually use. These secondary energy forms are usually referred to as energy carriers and they include, among others, electricity, various type s of combustible fuels, and thermal energy. The energy provided by energy carriers is consumed in a wide variety of human-designed systems and machines that are capable of (1) doing work that humans would rather not do, or cannot do with their own muscular effort, or (2) altering local environmental conditions so they are more suitable for the health and well-being of the human body.

When considering the various energy carriers that our modern industrial societies use, electricity is thought of as a high-end energy carrier. This is because it is relatively expensive to produce from primary energy resources, it can be transported with relative ease over long distances, it can be used to efficiently perform a wide variety of end-use tasks, because it can be used to generate other energy carriers, including combustible fuels and thermal energy, and because, for all practical purposes, it is essential for some end use applications, such as lighting. Fuels are also considered high-end energy carriers for many of the same reasons as electricity, although electricity has additional advantages that relate to its flexibility with respect to end-use applications and the ease with which it can be distributed to either large or small end-use devices. Low-grade thermal energy (usually this means the thermal energy of materials whose temperature is less than the boiling point of water) is considered a low-end energy carrier. It cannot easily be transformed into other energy carriers, it has a limited range of end-use applications, and it cannot be cost-effectively transported over long distances.

The charts below give a detailed breakdown of residential end-use energy applications in the United States for 2016. Other developed countries have similar residential energy use patterns. Surprisingly, a large portion of the energy used in our homes is in the form of low-grade thermal energy, and the same is true in our retail businesses and manufacturing facilities. With the exception of the transportation sector, other economic sectors, such as retail and manufacturing for example, have similar patterns of thermal energy usage. For the most part, the low-grade thermal energy that we use in different economic sectors is obtained either by burning high-grade fossil fuels or by using electricity that is generated from fossil fuels. In other words, the large quantity of low-grade thermal energy that we use is currently produced from high-end energy carriers. This is an obvious waste of precious high-end resources. Our need for low-grade thermal energy could easily be met by using the “waste heat” generated in other industrial processes, with the generation of electricity being a prime example of such a process.

Here it is worth noting that the low-grade thermal energy collected and stored locally by SEA cogeneration systems (SEA Cogen) could be used to displace the electricity and the fuels now being used to satisfy low-grade thermal energy needs. Doing so would significantly reduce the quantity of fossil fuels that we consume and it would also greatly decrease the amount of electricity that we need to generate. (In our homes and businesses, more than 40% of the electricity we use and more than 70% of the total energy we use is for low-grade thermal energy applications and low-grade heat transfer operations. That’s an unacceptable waste of high-end forms of energy.

2016 Residential Energy and Electricity Use (Source: US EIA)

2. LONG-TERM STORAGE OF LOW-GRADE THERMAL ENERGY

When an SEA cogeneration system is operating, thermal energy is removed from the solar cell arrays by coolant flowing through the cooling channels that support the arrays. The heated coolant can be circulated through borehole heat exchangers, thereby transferring thermal energy to underground thermal storage reservoirs (undisturbed layers of soil, gravel, and rock).  This energy, which is collected and stored locally, can be reclaimed as needed—after days, weeks or even months of storage—for end-use applications shown in the residential-energy-use charts above, or it can be used in drying operations or as process heat in industrial or agricultural operations.  

This proven technology is referred to as Borehole Seasonal Thermal Energy Storage (BSTES) and it is now being used successfully in a number of locations in northern Europe and Canada. The BSTES technology (e.g. DLSC) is extremely efficient, with coefficient of performance (COP) values of 30 being typical.  For every joule of electricity used in operating BSTES systems, 30 joules of thermal energy are delivered to the user. That’s why we shouldn’t think of low-grade thermal energy as being of low value. The true value of the low-grade thermal energy collected and stored by SEA systems should be measured in terms of the cost of the electricity and the fossil fuels that are displaced by its use.

3. LONG-TERM STORAGE OF ELECTRICITY

Regarding the electricity produced by our SEA cogeneration systems, some will be used as it is generated, some can be stored in batteries for the short term (overnight or for a few days), and the majority can be stored for the long term (season-to-season) in the form of various fuels.  These fuels, produced through a series of processes that begins with the electrolysis of water, can be stored until needed and then burned in heat engines to regenerate a portion of the electricity that was originally used to create the fuels.