Saving for a Rainy Day
Written by Susan E. Groh

You've been invited to spend the weekend at your cousin's new cabin in the Poconos. She and her husband, having decided to "live simply", have constructed their home far from the beaten path, with an eye to being as energy efficient as possible. They've installed solar cells and collectors for generating electricity and heating their home, but are still trying to decide on the best way to trap and store energy for use at night and on cloudy days. They had planned to construct a tank containing some substance that can absorb the energy of sunlight, and then use that energy to provide heat for their home. They've found some plans for how to distribute the heat from such a storage reservoir throughout the house, but are still unsure about what materials to use for the tank and that substance. They'd originally thought of having a steel tank full of water, but are now intrigued by a magazine article that discussed the use of "phase-change materials" to store energy.

They show you some tables that appeared in the article.

Table 1. Heat Storage Materials
Material Specific Heat Density Heat Capacity
  (Btu/lb/degF or
(lb/ft3) (Btu/ft3/degF)
Water 1.00 62 62
Steel 0.12 490 59
Copper 0.09 555 50
Aluminum 0.22 170 37


Table 2. Properties of Phase-Change Materials
Material Density Heat of Fusion Melting Temperature
  (lb/ft3) (Btu/lb) (degF)
Glauber's salt
Na2SO4.10 H2O
91 108 88 - 90
Na2S2O3.5 H2O
104 90 118 - 120
Paraffin 51 75 112
Calcium chloride
CaCl2.6 H2O
102 75 84 - 102

The problem here is that neither your cousin nor her spouse have much of a background in science, and they don't know what to make of the data in these tables, and so can't proceed with their decision making. They're hoping that you might be able to interpret this, and give them some advice about choosing a heat storage system (both the tank material and the substance to be kept in it) that will enable them to store as much heat as possible, within a reasonable physical space. They'd like to be able to store enough energy for three day's use; the article gives an estimate of about 480,000 Btu as the thermal energy needed to heat a house similar to theirs for a day.

What advice would you give?

Preliminary Questions:

  1. What do the data in these tables mean to you? Can you identify all of the terms and units? What information can you get from these?

  2. What sorts of assumptions might you need to make to get started on your decision? Are there other data you think you need here?

Last updated Feb. 9, 1999.
Copyright Sue Groh, Univ. of Delaware, 1999.