Lawrence Berkeley
National Laboratory
Environmental Energy Technologies
Indoor Environment Department
Energy Performance of Buildings Group

Decentralized Latent Storage in Buildings

Cooling of residential California buildings contributes significantly to electrical consumption and peak demand mainly due to very poor load factors in milder climates. The peak cooling load requires utilities to build, operate and maintain peak-power plants, and size their distribution network accordingly. For the building owner, the peak-cooling load determines the size of the equipment and the choice of the cooling source. Several steps can be taken to downsize the cooling systems and to allow switching to low-energy cooling sources:
· incorporate facades which provide an effective shelter from ambient conditions
· install highly efficient thermal distribution systems (e.g., hydronic systems)
· apply thermal conditioning by radiation rather than by convection
· provide thermal mass.

Large thermal storage devices have been used in the past to overcome the shortcomings of alternative cooling sources, or to avoid high demand charges. Buildings designed to make use of thermal storage include features which increase thermal mass. These may be used for storage only, or may serve both as storage and as structural elements. Several structural materials satisfy the requirements for sensible heat storage; these include concrete, steel, adobe, stone and bricks.

Latent heat storage uses a phase change material as a storage medium. This concept is particularly interesting for lightweight building construction. While undergoing phase change - freezing, melting, condensing, or boiling - a material absorbs or releases large amounts of heat with small changes in temperature. Phase change applications typically involve liquid/solid transitions. The Phase Change Material (PCM) is solidified when cooling resources are available, and melted when cooling is needed. PCMs have two important advantages as storage media: they can offer an order-of-magnitude increase in heat capacity, and for pure substances, their discharge is almost isothermal.

So far, only samples of PCM-treated wallboard exist. There are several approaches to treat wallboard with PCM material; however, none of these approaches has been tried in the industrial production process.
The manufacturing of phase change material (PCM) imbedded in gypsum board, plaster or other wall-covering material would allow the thermal storage to become part of the building's structure. This would permit the storage of high amounts of energy without changing the temperature of the room envelope. Since storage would take place inside the building where the loads occur, rather than outside, additional transport energy would not be required. Considering that more than 7 billion square meters of plaster board are being produced annually in the US, PCM-treated wallboard could have a significant impact on the utility peak. At the same time, it could help to moderate temperature swings and improve thermal comfort in homes.

Phase change materials can only store energy, but not remove it. In passive applications of structural thermal storage, the heat is being released into the room as soon as the room air temperature falls below the phase change temperature. This heat release mechanism keeps the surface temperatures of the room envelope at a high temperature level for a long time. This has certain advantages for the heat transfer mechanism during the discharge of the thermal storage.

Besides the passive application, treated wallboard could be coupled with a hydronic loop. Combining continuous discharge and phase change material allows the discharge of thermal energy storage without releasing the energy back into the conditioned space.

THERMAL PERFORMANCE
Physical Properties of Treated Wallboard
Table 1 shows the physical properties for wallboard with a homogenous distribution of PCM material as measured by Oak Ridge National Laboratory. Unfortunately, the same data is not available for wallboard treated with PCM encapsulated in pellets.

PCM Wallboard Characteristics

As all organic PCM will continue to burn in normal atmospheric conditions after igniting, potentially severe fire-hazards related to PCM-treated wallboard exist. Two tested methods have shown promising results in eliminating the fire hazard for treated wallboards: limiting the amount of PCM to 20%, and sequentially treating the plasterboard with PCM, and with an insoluble fire retardant.

Only ultra-pure paraffins melt and freeze sharply at a given temperature. Mixtures of PCM show a region of temperatures where melting takes place. Results from experimental studies and simulation exercises showed clearly that the treated wallboard does not act like an ideal storage material, which would melt and freeze at a specific temperature. Comparison between measured data and simulation results for the dynamic behavior of a stack of wall boards showed that the best agreement was obtained if the specific heat as a function of temperature was modeled by the typical triangular-shaped curve.

Performance of Phase Change Material
As the specific heat is taken as the temperature derivative of the specific enthalpy h, the specific heat as a function of temperature shows a discontinuity at the melting temperature.

At the melting point , the specific heat shows very high positive values. The thermal diffusivity is constant at high and low temperature levels, where we find the linear temperature regimes. While crystalline substances and eutectics show a discontinuous transition, many materials (e.g., mixtures) show continuous enthalpy curves as a function of temperature. This leads to a "mushy region" between the solid and the liquid phase.

Applications
Numerical studies show a significant reduction of indoor air temperature for houses/buildings without mechanical cooling and downsizing potential as well as reduction of peak-load power and energy consumption for houses/buildings with mechanical cooling equipment. Latent thermal storage only performs well if the storage is periodically being discharged either by natural cooling sources (e.g., night cooling) or by mechanical cooling sources during the time of lower cooling load (e.g., operaqtion after occupancy hours or early morning).

Literature
Feustel, H.E. and C. Stetiu
Thermal Performance of Phase Change Wallboard for Residential Cooling Application
Lawrence Berkeley National Laboratory, Report LBL-38320, 1997

Stetiu.C. and H. E. Feustel
Phase-Change Wallboard and Mechanical Night Ventilation in Commercial Buildings
Lawrence Berkeley National Laboratory, 1998

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