- For many households and businesses this will be space heating and cooling together with water heating.
- Using the same piece of equipment to perform several functions is one way to keep the equipment purchase cost and installation cost down.
- Aim to use a minimum of high-priced energy and use it mostly in off-peak periods when its price is likely to be discounted.
An example of how these objectives may be achieved
A reverse-cycle air conditioner is just another name for a heat pump. Heat pumps use electrical energy to "pump" heat from one temperature to another temperature. Refrigerators contain a heat pump that "pumps" heat from the inside of the refrigerator to the outside.
The amount of electrical energy a heat pump uses is less than the amount of heat it is able to "pump". On a cool day a heat pump in a reverse-cycle air-conditioner may be able to pump 5 kilowatt-hours of heat energy into your home to warm it up from the air outside using only 1 kilowatt-hour of electrical energy. On a very cold day it may only be able to pump 3 kilowatt-hours of heat energy into your home from the air outside using the same amount, 1 kilowatt-hour, of electrical energy.
Heat energy can be stored in a "phase change material". This is any substance that melts, changing from a solid to liquid at some constant temperature, absorbing a quantity of heat energy while it is melting. Ice that melts to water at zero degrees Centigrade (32 degrees Fahrenheit) while absorbing heat energy is an example of a phase change material. The same phase change material solidifies at its melting point temperature - changing from water to ice - at zero degrees Centigrade (32 degrees Fahrenheit) while releasing heat energy.
Cooling in summer
A quantity of a solid phase change material that melts while absorbing heat at say, 18 degrees Centigrade (64.4 degrees Fahrenheit) , can be used to cool a home or business office by absorbing heat during hot days.
Moving #airconditioners #hvac to off peak power using #phasechangematerials compact cold storage http://t.co/zfn6nz6G8Z
— Phase Energy (@PhaseEnergy) May 23, 2014
If the overnight minimum temperature is below 18 degrees Centigrade (64.4 degrees Fahrenheit) the phase change material can be cooled and solidified while releasing heat energy with no electrical energy being needed. It would then be ready to provide cooling the next day, again with no electrical energy being needed.
To be reliable a heat pump could ensure the phase change material solidifies overnight on slightly warmer nights by "pumping" heat from the phase change material into the outside air. The amount of electrical energy needed to do this will be much less than pumping heat into the outside air when the temperature is much hotter in the daytime. The price of the electricity will hopefully be less overnight than during daytime hours.
The heat pump should also be able to pump heat into the phase change material during the daytime from the air inside your home or business office to ensure reliable cooling. Very little energy will be needed to do this compared to pumping heat into the outside air during very hot days.
An example of a system that does this is marketed by Phase Change Products Pty Ltd (PCP). There is an outline in the post "Having your air conditioner and low peak demand too".
Heating in winter
A quantity of a liquid phase change material that solidifies while releasing heat energy at say, 30 degrees Centigrade (86 degrees Fahrenheit) , can be used to warm a home or business office during cold days and nights.
Even on cold days with a maximum temperature of 15 degrees Centigrade (59 degrees Fahrenheit), the temperature in a closed container that allows sunlight to enter can quickly warm to a much higher temperature.
|The Australian National University, Solar Thermal Group|
A number of individual solar air heating modules
can be arranged end-to-end or side by side
The Australian National University has developed a low-cost module that can double as both part of the roof of a home or business office and absorb solar energy at a temperature greater than the outside air temperature.
Bluescope has recently unveiled a commercial roofing product that delivers this concept.
Clever Australia: Bluescope unveils “world first” built-in #solar roof that generates both heat and power http://t.co/las7USxyE2
— RenewEconomy (@renew_economy) June 13, 2014
ARENA joined Parliamentary Secretary @bobbaldwinmp and BlueScope Steel to ‘switch on’ a new #solar roof in Sydney. pic.twitter.com/IW8ZiNA9dV
— ARENA (@ARENA_aus) June 13, 2014
The amount of heat energy that can be used from this solar thermal energy collector is greatest when the temperature of the air inside it is relatively low. As the absorber is allowed to heat up to higher and higher temperatures, an increasing proportion of the absorbed solar thermal energy is re-radiated back into the outside air. Eventually it reaches an equilibrium temperature at which the energy entering is equal to the energy being re-radiated. At this "stall temperature" no energy is available to pump into a phase change material - without lowering the temperature in the solar thermal collector.
A heat pump is needed to reliably add heat energy to the quantity of phase change material from the solar absorber during sunlight hours. The amount of electrical energy needed to do this will be considerable less than what the heat pump would use to pump the heat energy from the much colder air outside your home or business office.
To warm your home or business office overnight, a heat pump would need very little electrical energy to move heat energy from the quantity of phase change material that solidifies while releasing heat energy at 30 degrees Centigrade (86 degrees Fahrenheit).
Heating water, both summer and winter
The same quantity of liquid phase change material that releases heat energy while solidifying that is used for space heating in winter can also be used for heating water in a hot water storage tank. A heat pump can pump heat energy from the liquid phase change material into the hot water tank overnight, taking advantage of the lower-price electricity available during off-peak hours.
This will use less energy than a heat pump that was pumping heat from cold air overnight and be cheaper than pumping heat from warm air in the daytime when electricity prices are higher.
One Heat Pump for All Functions
A purpose-designed heat pump intended to perform each of the functions would provide the benefits of low capital cost and low installation cost. When being used to transfer heat to and from phase change materials it should be highly efficient for these small temperature lift applications.
Analysis of the design and operation of such a heat pump is in the articles:
- "Empirical Modeling of a Rolling-Piston Compressor Heat Pump for Predictive Control in Low Lift Cooling" by N.T. Gayeski, Ph.D.; T. Zakula; P.R. Armstrong, Ph.D.; and L. K. Norford, Ph.D. - ASHRAE, and
- "High efficient heat pumps for small temperature lift applications" by I. Wyssen, L. Gasser, M. Kleingries, Dr., and B. Wellig, Prof. Dr., Lucerne School of Engineering and Architecture, Horw, Switzerland