Cold Weather Passive Assist Refrigeration
Concepts for Sustainable Living
The following are not products, but rather are concepts and ideas for living sustainably in today’s world.
Sun Frost Energy Efficient Shower
The Problem
The design features of a conventional shower minimize thermal comfort, maximize energy use and water consumption while exacerbating moisture problems. Most of these negative effects are caused by evaporation. As small droplets of water fall from the showerhead, the high surface area and velocity of the particles of hot water experience conditions, which maximize evaporative cooling of the droplets. Water saving showerheads that produce exceptionally small water particles can actually further increase evaporation. Water Particles can be cooled by as much as 30ºF as they fall from the showerhead to the ground. Evaporation is also accelerated by convective air currents and low humidity. With the shower curtain in place, a shower area can act as a thermal chimney. Cold air enters from below the curtain; then rises as it is heated by the walls of the shower stall, falling particles of hot water, and the shower’s occupant. The humidity of the moving air is kept low by condensing its moisture load on cold surfaces in the bathroom. This often results in mold growth, rotting walls or ceilings, and wet insulation; a problem that can be particularly acute in straw bale construction.
Bathroom moisture is often controlled by the use of an exhaust fan. The fan sucks out the moisture-laden air, which is then replaced by drier outside air. During the winter, the moisture level in outside air is particularly low, further accelerating the rate of evaporation. In addition, heat is required for the fan’s make up air.
Evaporation is also the main source of heat loss from our skin. Lower humidity will result in increased heat loss from the shower’s occupant and require turning up the hot water to maintain comfortable conditions. If a window is opened to control moisture, the temperature of the bathroom will be lowered, the shower’s occupant will have to then turn up the hot water to stay comfortable. Causing the bathroom to turn into a steam room as moisture in the humid air leaving the shower condenses in the cooler room air.The walls of a bathroom are often covered with high thermal mass materials, such as ceramic tiles. The tiles will typically be below skin temperature resulting in additional heat loss via radiation and convection. The shower’s occupant will then again compensate for this heat loss by turning up the hot water even further.
The Solution
There is a simple solution to this myriad of related problems; preventing airflow to and from the showing area is the key, in other words sealing the shower stall. Additional benefits can be obtained by the incorporation of insulated low thermal mass walls that heat up rapidly. The result is a shower stall where air currents are minimized, humidity is increased and the air is warmer. All these effects will increase thermal comfort and reduce energy and water consumption. Another benefit is that with the air in the shower warmed, the water vapor present will not be condensing in the air, eliminating steamy conditions.
If the walls of the shower have a fairly low thermal mass, the shower stall will heat up rapidly so a comfortable shower can be taken even in an unheated bathroom. We constructed a shower stall for Home Power magazine. The stall has very low thermal mass; the walls are covered with “Reflectics” (aluminized bubble wrap). The top of the shower is covered with a clear plastic bubble, see photo. The shower is located in an unheated greenhouse. During the winter, when greenhouse temperatures dip to 38ºF, you can still take a comfortable shower.
A fiberglass tub/shower combination in my home was sealed with plywood and Plexiglas. This configuration allows for a comfortable shower when the air in my bathroom dips to 50ºF. After showering, I can dry off in the warmed up shower. A vent is opened in the ceiling to dry off the interior. This arrangement allows me to take a comfortable shower in a cold bathroom while using only 1/2 gallon of water per minute. Many showers use a much as 3 gallons of water per minute.
Application In Solar Homes And Developing Countries
This energy efficient shower combines effectively with solar heating, since it allows for a fairly comfortable shower when water temperatures dip to 90ºF (Typical water temperature for a shower is 105ºF). In tropical countries showers are often unheated. The water is generally not quite warm enough for a comfortable shower. To boost the temperature of the water, an electrical heater located at the showerhead is often used. The sealed shower described here could move the water into the comfort zone without using inefficient and costly electrical heating.
Task Air-conditioning For Sleeping
The Sun Frost “Sleep Genie”
In a hot humid climate, keeping comfortable by passive strategies is often difficult and sometimes not possible. Passive strategies, such as high ceilings and large operable windows to increase ventilation, can make conventional air conditioning more difficult because of increased heat gains. Sun Frost has developed an air-conditioner to provide comfortable sleeping conditions even in an airy uninsulated building. The strategy to accomplish this feat is to create a relatively small, insulated space around the bed. This space can then be air-conditioned and dehumidified with a very small air-conditioner. A diagram of the air-conditioner and enclosure is shown in the accompanying figure. Here the structure is installed over a small single bed. Since cold air is heavier than hot air, the space above the occupant’s head can be left open.
The compressors we are currently using to power the cooling system can be run off either a 12-volt or a 24-volt battery bank. We will also be building a 120-volt model. The typical load in a 12-volt system will be about 6 amps or 72 watts. With 8 hours of sleep the nightly electrical load at 12 volts will be about 72 amp-hours or 600 watt-hours. To power this air conditioner with photovoltaic panels, a 280-watt array would typically be required.
In addition to homes, this concept could find wide application in long-haul trucks with sleeper compartments. Currently, drivers idle their trucks all night to keep cool in hot weather. This burns up about 1 gallon of diesel fuel per hour and causes a great deal of noise and air pollution. Idling trucks all night is becoming illegal in a great many states for these reasons. With Sun Frost’s “Sleep Genie”, idling would not be necessary because the air-conditioner could be run off the truck’s starting batteries.
In some tropical countries like Singapore, many of the buildings are concrete and use no passive strategies for cooling. As a result of poor design, heat from sun-baked walls will begin to penetrate into an apartment several hours after sunset creating the most uncomfortable conditions as you are getting ready to go to bed. In Singapore, air-conditioners are often only used at night to make a bedroom more comfortable. Instead of cooling a high thermal mass, uninsulated space, a task air-conditioner like the “Sleep Genie” would be a much more energy efficient solution. Of course, it would make the most sense to combine strategies, incorporate the “Sleep Genie”, insulate the building and put windows in the most appropriate locations with effective shading devices and then use the “Sleep Genie” when necessary. “Task” air conditioning is a simple strategy for cutting the air conditioning electrical loads for the 1/3 of your day you are off your feet and making solar cooling more practical.
Cold Weather Passive Assist Refrigeration
If you live in a cold climate, your refrigerator is often running when the temperature outside is colder than it is in your refrigerator. A simple way of using the “coolth” from the outside to cool your refrigerator is by incorporating a heat pipe. A heat pipe is a passive device that transfers heat by evaporation and condensation. Steam heat and steam cooking vegetables are examples of heat pipes in use. Absorbed heat boils water, the steam then condenses and releases its heat while heating your house or cooking your food, the condensed water then drains back to the boiler.
A heat pipe can be used to transfer heat from a refrigerator compartment to the outside. When the temperature is colder outside than in the refrigerator, the liquid in the refrigerator boils, absorbing heat and cooling the refrigerator compartment. The refrigerant then condenses outside and runs back to the refrigerator (see diagram). The pressure in the tube changes with temperature, so the liquid always boils when the temperature is colder outside than in the refrigerator. This device acts as a one-way thermal valve, if the temperature is hotter outside than in the refrigerator heat is not transferred into the refrigerator.
Refrigerators we manufactured incorporating a heat pipe also contain an active cooling system. When the temperature outside warms and the refrigerator can no longer be passively cooled, a thermostat with a preset temperature will turn on the compressor and cool the refrigerator compartment. Installation required only drilling a _ inch hole for the heat pipe running from the refrigerator to the outside.
In the mid 1980’s we built about ten of these hybrid refrigerators. The models without a freezer compartment coupled very effectively with a solar system. Since during the winter when available sunlight is limited, the refrigerator is passively cooled and requires no energy to run. With our RF16, the freezer compartment consumes about 60% of the total energy consumed by the unit. During the winter, the heat pipe would decrease energy use by 40%. One of the passively cooled refrigerators is located at the Rocky Mountain Institute in Colorado, where winters are long and passive cooling is effective. Although these refrigerators operate effectively, sales were limited and we no longer manufacture that model.