Heat transfer is most efficient by convection, then by conduction; radiation is the least efficient and slowest means of heat transfer.
It depends on the medium. If it is solid then conduction will be the fastest and in liquids and gases, you can use convection. However, these two modes of heat transfer cannot take place in a vacuum as they require particles and a vacuum is empty.
Geothermal Heat Pumps
Even a few feet underground, temperatures remain between about 45 to 60 degrees all year. Either water or a refrigerant solution circulates through the loop, picking up heat or releasing it as necessary. Geothermal systems provide the most efficient type of heating.
Plate heat exchangers are up to five times more efficient than shell-and-tube designs with approach temperatures as close as 1°F. Heat recovery can be increased substantially by simply exchanging existing shell-and-tubes for compact heat exchangers.
Conduction is heat transfer through stationary matter by physical contact. (The matter is stationary on a macroscopic scale—we know there is thermal motion of the atoms and molecules at any temperature above absolute zero.)
Convection can transport heat much more efficiently than conduction. Air is a poor conductor and a good insulator if the space is small enough to prevent convection. Convection often accompanies phase changes, such as when sweat evaporates from your body.
Heat transfer is most efficient by convection, then by conduction; radiation is the least efficient and slowest means of heat transfer.
By contrast, counter flow is significantly more efficient and, depending on the flow rate and temperature, the heat transfer performance could be up to 15% more efficient, possibly enabling a smaller heat exchanger to be used, saving space and money!
A plate heat exchanger is the lowest cost option because it can achieve high heat transfer coefficients — with pure counter current flow — giving the most efficient heat transfer and lowest surface area.
Heat engines often operate at around 30% to 50% efficiency, due to practical limitations. It is impossible for heat engines to achieve 100% thermal efficiency () according to the Second law of thermodynamics.
Electric resistance heating is 100% energy efficient in the sense that all the incoming electric energy is converted to heat. However, most electricity is produced from coal, gas, or oil generators that convert only about 30% of the fuel's energy into electricity.
The average cost to install a geothermal heat pump typically ranges from $10,000 to $30,000, depending on factors like the size of the system, property location, and installation complexity. Horizontal loop systems tend to be less expensive, while vertical systems, which require deep drilling, are on the higher end.
Simply put, natural gas fired units are the least expensive to operate of all central heating units. When efficiency is considered, both gas and oil boilers are less efficient than electric ones. Of course the cost of electricity continues to increase so the overall cost may not be less than fossil fuel fired units.
Conduction is the most significant means of heat transfer within a solid or between solid objects in thermal contact. Fluids—especially gases—are less conductive.
Heat exchanger efficiency is calculated by comparing between the real and perfect performance. Though perfect performance is calculable, it is unfeasible in the real world, because of the thermodynamic limitation that states nothing can be 100% efficient.
Heat energy will always be transferred from a hotter to a colder object.
The ideal exchanger has an efficiency of 100%.
A real heat exchanger does not achieve such efficiency. However, a plate heat exchanger is the most efficient heat exchanger commonly available. Typically, efficiencies greater than 90% are achieved.
Occurrence of flooding in Reactor Systems
This limit of the stable countercurrent flow is known as flooding. There are several locations within the reactor cooling system where gravity drainage of liquid can be impeded by upward flowing vapor and potentially result in an undesirable flooding situation.
Typically, a countercurrent pattern in a shell and tube heat exchanger is the most thermally efficient option.
The maximum amount of heat or mass transfer that can be obtained is higher with countercurrent than co-current (parallel) exchange because countercurrent maintains a slowly declining difference or gradient (usually temperature or concentration difference).
Cocurrent flow: Both fluids flow in the same direction. This is the simplest and most efficient type of heat exchanger but does not allow for temperature cross (the outlet of cold fluid to be hotter than the outlet of hot fluid, or vice-versa).
The more light the object absorbs, the more heat absorbed since light is energy. If you consider it a color, black absorbs the most heat. A black object absorbs all wavelengths of light and reflects none. Objects that are white, on the other hand, reflect all wavelengths of light and therefore absorb the least heat.
Heat is transferred to and from objects -- such as you and your home -- through three processes: conduction, radiation, and convection.