March 12th, 2009

Ann Vole

Heat pumps for ovens (repost from Envirolink)

I posted this stuff on a forum called Envirolink.org and am transferring what I wrote there before it gets erased as an old post:

me: Refrigerators and freezers use heat pumps where some energy is used to operate a heat pump that moves heat from the inside of the refrigerator or freezer and puts that heat plus the heat generated by the energy used in the heat pump into the "condenser" coils outside the refrigerator or freezer. Heat pumps can also be used to heat or cool homes and the heat or cool that is not needed can be either put into the outside air or better, can be put into the ground or a body of water. Some heat pumps made for heating your house can also heat your water for your hot water needs. Just recently, some heat-pump operated hot water tanks have been developed. I like the idea but solar hot water heaters are also available at a lower cost and do not require electricity for the heat pump. Good insulation and passive heating and cooling techniques can also reduce the need for a heat pump system for heating and cooling. Running a range or oven or stove off of solar is a bit trickier due to having to store heat well above the boiling point of water so a heat-pump-operated cooking device would be the ultimate use for a heat pump in my opinion. Unfortunately I have yet to find a heat pump powered cooking device. I might have to invent one and incorporate it into a refrigeration system and maybe hot water unit so solar input can be added and all your kitchen needs will be integrated in a single unit (including an ice maker).

someone else: What about the new ranges that are.... magnetic? I think it is something like that

me: they are called "Magnetic Induction" stoves. To cook food you need heat and the closer you get the heat to the food, the less of the heat gets wasted heating the air in the room. They tried flat surface cooking surfaces (instead of the coils) and got a better transfer of heat to the pan (more efficient then coils) because more of the heating surface was touching the pan. Then they tried radiant heat where the stove surface does not get hot but rather emits infrared light that shines on the bottom of the pan and turns into heat at the bottom of the pan (more efficient then using a hot surface). Magnetic induction actually turns the metal in the pan into the "secondary" side of an electrical transformer thus creating an electrical current inside the metal of the pan and that then heats the inside of the metal pan which is more efficient then infrared that might shine into the room and might reflect off the pan and also has to move through the pan to get to the food. Microwave ovens work my making radio waves that are just the right frequency for molecules like water to get electrically affected and effectively spin and heat up. This makes the heat source to be the water inside the food which is the most efficient way of moving the electrical energy into heat in the food. The problem is that to brown your food, you need to get hotter then the boiling point of water and if water molecules turned to steam are your heat source, you will never get hotter then the boiling point of water.

Now that I told you how to get close to 100% of the electrical energy into heat inside the food, how about putting 400-500% of the electrical energy used into heat in the food? For example, a 500 watt microwave on for 1 minute will heat a cup of water from room temperature to almost-too-hot-to-drink temperature but a heat pump will take about 375 watt-minutes worth of heat out of the air and cram it into the condensing coils and use about 125 watts of power for one minute to do so resulting in the same amount of energy being put into the cup of water (500 watt-minutes) and resulting in the same temperature rise in the cup but with 1/4 the power usage (and the room gets cooler... or the refrigerator gets cooler if the "evaporator" coils are in the refrigerator)

a third person: The problem there, my dear friend, is that heat "pumps", whether they be air conditioners, refrigerators, or actual heat pumps designed to heat a home, exchange heat from one body of air to another. Try the same with water and you'll find that more energy input is required because its heat capacity is higher.

me: yes water has a high heat capacity... I am comparing heating a glass of water with different methods... it all works the same no mater what you are putting the heat into. As far as heating water, those heat pump powered hot water tanks use several times less electricity then an electric element hot water tank of the same capacity and use profile. The only difference is their heat pumps use a different refrigerant (then refrigerators or house heat pumps - geothermal) because of the different temperature range it is designed to work in (move room temperature heat into a hot water tank) and for cooking I would need a different refrigerant again to have the refrigerant condense at 140 C or so (well above the boiling point of water) in order to fry food with the thing.

me (again): I think I missed your point in my first response (even though you italicized the "air"). I have been doing most of my heat pump research with geothermal heat pumps and a vast majority of them use liquid to move the heat and cool around because refrigerants are expensive and dangerous. Moving heat is best done with a liquid with as high a heat capacity as possible so down south they can get away with using water but up north in the winter, the heat pump is sucking so much heat out of the liquid that runs through the ground that the liquid is often dozens of degrees below freezing when returned to the ground. They use various liquids but they have to make sure that it will not poison people if a pipe breaks and leaks into the well water aquifer. As you can see, heat pumps used for geothermal do fine moving heat from one liquid to another (radiant heat systems) or often from the liquid pumped in the ground to the air heating the house.

a fourth person: Ann do you do much solar cooking?

me: no, I did some solar cooking for fun experiments (boiled eggs on a cloudy day in -35 C weather) but the key to green cooking appliances people will actually buy and use, it has to work fast and reliable all the time (even at night) and it cannot look ugly on the house if you are in a city (which may not have very much solar input to start with). High efficiency and buying "green" electricity (that the utilities make with wind or solar projects) would be the only viable options for a vast majority of city dwellers to reduce their carbon footprint or decrease their use of non-renewable resources (or foreign oil). When it comes to heat, heat pumps move a lot more heat energy then they use to pump it (and the energy they use to pump also gets added to what they move) so is clearly the best option for heat needs. For cooling, heat pumps have proven to be the only option (refrigerators, freezers, air conditioners are all heat pumps). Insulation and heat storage are the only further improvements on the heat pump designs.

MY way of energy self-reliance is better: I am learning how to prepare and enjoy foods that require no cooking and no refrigeration so you may see me promoting heat pumps on one hand (for the majority of people who need to cook and refrigerate their foods and people trying to heat and cool a "normal" house) but on the other hand, heat pumps still need electricity so I am doing everything I can to eliminate the need for heat pumps in my building designs. As far as solar cooking, I hope to build a unit that gathers solar heat at about 200-300 C (well above the hottest a home oven will get) using tracking solar and vacuum-containing Plexiglas tubes around the black collector tubes to heat oil (oil pumps well and can handle the temperature) and pump it through a heat storage of dirt under the house that is insulated (mineral wool). The oil can then be pumped through coiled pipes to make what looks like a regular electric stove or oven but the heat is solar source and it is hot oil flowing through the tubes instead of electricity.