Seasonal energy storage

ASHRAE Journal, Jan, 2009 by Kurt Roth, James Brodrick

Most buildings meet thermal loads using equipment and systems that generate or remove heat when building loads exist. Thermal energy storage (TES) enables buildings to meet heating and cooling loads using energy produced at other points in time.

TES can be designed for storing and providing energy on three basic timescales: diurnal, weekly, and seasonally. (1) Cold storage tank systems using ice or chilled water are examples of diurnal storage systems, i.e., they produce ice or chilled water in anticipation of cooling loads within the next 24 hours.

In contrast, seasonal thermal energy storage (STES) enables a building to use heat collected during the summer to heat the building in the winter, or to use snow collected during the winter to cool the building in the summer. Relative to diurnal storage systems, STES requires a much larger total size of the TES system, while the rate of charge and discharge varies much less with timescale.

STES systems consist of several components, including a heat (or coolness) source, heat exchange system, thermal distribution system, thermal storage medium, and thermal loads (Figure 1).

Ideally, an STES saves low-cost heat that would otherwise not be used. Thermal energy sources used by STES systems include solar thermal (typically low-temperature collectors), industrial waste heat, excess heat from district energy systems, snow and ice, and seawater. (2,3)

Examples of thermal loads met by STES are commercial and multifamily residential building space and water heating, space heating for greenhouses, roadway deicing/snow melting, and building space cooling. (1,2,4) In general, STES tends to be most attractive for applications with significant heating or cooling loads that are offset by several months from the peak availability of thermal resources.

For example, solar thermal collectors in Northern climates collect much larger quantities of heat during the summer than in the winter due to the longer solar days in summer, while space heating loads peak in winter. Similarly, cool STES also works best in climates where large quantities of snow and ice can be harvested during the winter to provide space cooling during the summer. Consequently, STES energy savings and economics tend to be more favorable in colder, Northern climates.

Several types of STES are used, and the most common systems, namely aquifer thermal energy storage (ATES) and borehole thermal energy storage (BTES), store heat in the ground. Both take advantage of the fact that deeper than 10 m to 20 m (33 ft to 66 ft), the ground and groundwater temperatures vary little over the course of the year (4) to reduce thermal losses.

ATES systems transfer heat to and from groundwater in aquifers via wells drilled from the surface into the aquifer. Often, the wells are grouped separately, as warm and cold, to provide heating and cooling. (5) Favorable environmental characteristics for ATES installations include high ground porosity levels and significant water content around the wells to enable effective heat transfer between the wells and the aquifer, and low ground water flow through the aquifer to avoid convection of the stored thermal energy away from the wells. (4,6) * Because these qualities fundamentally impact the viability of ATES, the aquifer must be characterized before initiating ATES projects. (4,7) Both lower (10[degrees]C-40[degrees]C [50[degrees]F-104[degrees]F]) and higher (40[degrees]C-150[degrees]C [104[degrees]F-302[degrees]F]) temperature systems exist. (4) Although higher temperature systems have a greater storage capacity per volume, they have greater thermal losses and may also experience more problems with mineral precipitation. (1)

BTES systems consist of many boreholes (0.15 - 0.2 m [0.5 ft to 0.66 ft] wide) (6) drilled into the ground at depths ranging from 35 m (8) to 200 m (6) (115 ft to 656 ft) deep. A pump circulates a fluid (typically water or a water-glycol mixture) through pipes buried in the boreholes that transfer heat to and from the boreholes. After drilling the boreholes and installing the pipes in the boreholes, the borehole is back-filled, often with a material to enhance thermal conductivity, such as water, sand, or bentonite clay. Many BTES installations use closed systems, i.e., with a continuous pipe loop (U-pipe), while some use open systems that inject water at the bottom of the borehole and extract it near the top (but below the local water table). Desirable ground characteristics for BTES include high specific heat and thermal conductivity, as well as low groundwater flow. (5,6)

Both ATES and BTES require suitable ground conditions that do not always exist. Consequently, people have worked to develop other STES concepts.

Pit TES transfers heat to and from water (with or without gravel) stored in an excavated pit, with the top surface usually near or at the ground surface to reduce excavation costs.

Usually, all sides of the pit are insulated to mitigate thermal losses, particularly from the top, and the sides are made of concrete with liner to prevent water (both liquid and vapor) migration from the tank. (6,9) As with BTES, plastic pipes throughout the pit transfer heat to and from the pit. The typical sizes of storage tanks range from 100 [m.sup.3] to 10 000 [m.sup.3] (3,500 [ft.sup.3] to 353,000 [ft.sup.3]) for underground and partly buried tanks to 1000 [m.sup.3] to 1 million [m.sup.3] (35,000 [ft.sup.3] to 35.3 million [ft.sup.3]) for pit storage. (3)