Lesson 8b: Cooling and Heating
The following pages in Section B of Lesson 8 will cover cooling and heating.
Heat Movers
So far, we have discussed systems in which a fuel is burned and heat is produced and delivered into the home. In these systems, we buy all the energy, and (depending on the system) we reject or lose some heat to the surroundings, reducing the efficiency.
One of the ways in which we can improve the heating efficiency is to make use of the heat that is available outside, even on a cold winter day. On a cold winter day with outside temperature at 30ºF, the air still has more energy compared to air at 10 ºF or 5ºF. Air at any temperature above absolute zero (0ºK or -273 ºC) will have energy.
The higher the temperature of air, the higher its energy content. This energy can be transferred to the interior.
Heat Pumps
Under natural circumstances, heat only flows from high temperatures to low temperatures. In order to move heat from a low temperature environment to a high temperature environment, work needs to be done (or rather energy needs to be spent).
A device that moves the heat from a low temperature environment to a high temperature environment is called a heat mover.
An example of a heat mover is a heat pump. A heat pump is a heating/cooling system and also a forced-air system. Cooled (and sometimes humidified or electronically cleaned) air is usually delivered through the same ductwork and registers used by heated air.
A heat pump uses air-conditioning principles to extract heat from one place and deliver it to another, and vice versa. In addition to expelling heat from indoors, the system can be reversed to heat the home in the winter. Thus, a heat pump is a device that moves heat from a low-temperature to a high-temperature environment with the help of work that is put in.
Heat pumps are classified based on the low-temperature heat source:
- Air-source heat pump or Air-to-air heat pump. Heat is transferred from the low-temperature air outside to the high-temperature interior.
- Ground-source heat pump or Ground-to-air heat pump. The earth is used as a heat sink in the summer and a heat source in the winter; the pump relies on the relative warmth of the earth for its heating and cooling production.
- Water-source heat pump or Water-to-air heat pump. Heat is transferred from low-temperature water outside (from a pond or a lake) to a high-temperature interior.
An air conditioner is a cooling system and also a forced-air system. It runs on electricity and removes heat from the air with basic refrigeration principles.
Air-Source Heat Pump or Air-to-Air Heat Pump
An air-source or air-to-air heat pump can provide both heating and cooling.
- In the winter, a heat pump extracts heat from outside air and delivers it indoors.
- On hot summer days, it works in reverse, extracting heat from room air and pumping it outdoors to cool the house.
Nearly all air-source and air-to-air heat pumps are powered by electricity. They have an outdoor compressor/ condenser unit that is connected with refrigerant-filled tubing to an indoor air handler. As the refrigerant moves through the tubing of the system, it completes a basic refrigeration cycle, warming or cooling the coils inside the air handler. The blower pulls in room air, circulates it across the coils, and pushes the air through ductwork back into rooms.
When extra heat is needed on particularly cold days, supplemental electric-resistance elements kick on inside the air handler to add warmth to the air that is passing through.
Instructions: Click on the hot spots below to find out how the heating cycle of an air-source heat pump works:
In the winter, a heat pump extracts heat from outside air and delivers it indoors. In the summer, the heat pump extracts heat from room air and pumps it outdoors to cool the house.
Instructions: Observe the heating and cooling cycles of a heat pump.
Heating Cycle
Cooling Cycle
The Balance Point
As we have learned, air-source and air-to-air heat pumps work by extracting heat from the outside air. These heat pumps require a backup system to supplement their heating ability when the outdoor temperature gets below a certain temperature.
As the outdoor temperature drops, the heating requirement of the house increases and the output of the heat pump decreases. At some point, the temperature of the home’s heating requirement and the heat pump output match. This temperature is called the balance point and usually falls between 30-45 degrees Fahrenheit. For any temperatures below the balance point, supplemental heat will be required.
To locate the balance point, the heating requirement (BTUs/h) of the house and the heat pump output (BTUs/h) are plotted against the changes in outside temperature. The place where the home heating requirement and heat pump output lines cross is the balance point.
Take a look at the graph of the Balance Point.
Efficiency of a Heat Pump
Efficiency of a heat pump is measured using a term Coefficient of Performance (COP), and it is the ratio of the useful heat that is pumped to a higher temperature, to a unit amount of work that is put in. We will look at COP in terms of air-source heat pumps.
A general expression for the efficiency of a heat engine can be written as:
Using the same logic that was used for heat engines, this expression becomes:
Where, Q Hot = Heat input at high temperature and Q cold= Heat rejected at low temperature. The expression can be rewritten as:
Note: Thot and Tcold must be expressed in the Kelvin Scale.
Air-Source Heat Pump or Air-to-Air Heat Pump Examples
Example 1
Calculate the ideal coefficient of performance (COP) for an air-to-air heat pump used to maintain the temperature of a house at 70 °F when the outside temperature is 30 °F.Solution:
First, convert the Fahrenheit temperatures to Celsius temperatures using this formula:
Next, convert the Celsius temperatures to Kelvin temperatures by adding 273.
Finally, use the formula from the previous screen to solve for the COP.
The example above shows that for every watt of power we use (and pay for) to drive this ideal heat pump, 13.3 W is delivered to the interior of the house and 12.3 from the outside (we don’t pay for this). This seems to be a deal that one cannot refuse. However, the theoretical maximum is never achieved in the real world. In practice, a COP in the range of 2 to 6 is typical. Even with this range, it is an excellent choice, because for every watt of power that we use, we transfer 1 to 5 additional watts from outside.
Example 2
Compare the ideal coefficients of performance of the same heat pump installed in State College, PA and Ann Arbor, MI when the inside temperature of a house is maintained at 70°F at both locations and the outside temperatures on a given day were 40°F and 15°F at State College and Ann Arbor, respectively.
| State College, PA | Ann Arbor, MI |
|---|---|
| Thot= 70 ºF = 21 ºC = 294 K | Thot= 70 ºF = 21ºC = 294 K |
| Tcold = 40 ºF = 4 ºC = 277 K | Tcold = 15 ºF = -9.4 ºC = 264 K |
| =17.3 | = 9.8 |
During a heating season, the heat pump's efficiency increases on mild days and decreases on cold days.
Ground Source (Geothermal) Heat Pumps
Ground-source or geothermal heat pumps (GHPs) are similar to the air-source heat pumps, except that the source of heat is the ground instead of outdoor air.
Instructions: Where does the heat in the ground come from? Press the “play” button and then answer the questions that follow.
As you observed in the animation, the earth absorbs and stores energy from the sun as heat, resulting in underground temperatures that range between 40–80ºF, depending on the location. These temperatures, which are located below the frost line (which is generally 4–5 feet in Pennsylvania), remain constant throughout the year.
The geothermal heat pumps (GHPs) use the earth as a heat sink in the summer and as a heat source in the winter, and therefore rely on the relative warmth of the earth for their heating and cooling production.
Through a system of underground pipes, they transfer heat from the warmer earth to the building in the winter, and take the heat from the building in the summer and discharge it into the cooler ground. Therefore, GHPs do not create heat; they move it from one area to another.
Operating Principle
The GHP system operates much like an air-source or air-to-air heat pump, except that:
- The outside tubing is buried to extract or discharge the heat in the ground;
- The compressor/condenser unit is inside rather than outside the house.
The GHP system also has additional valves to allow heat-exchange fluid (refrigerant) to follow two different paths: one for heating and one for cooling. The GHP takes heat from a warm area and exchanges the heat to a cooler area, and vice versa.
Instructions: Press the play button to see how a geothermal heat pump operates.
Classification of GHPs
Closed-Looped Systems
Horizontal
The horizontal type of installation is generally most cost-effective for residential installations, particularly for new construction where sufficient land is available. It requires trenches at least four feet deep.
Horizontal systems come in two types of layouts, the two pipes method and the slinky method.
Two Pipes
The most common horizontal layouts include:
Two Pipes Layout (Option 1) - One pipe buried at six feet, and another pipe buried at four feet.
Two Pipes Layout (Option 2) - Both pipes placed side-by-side at five feet in the ground in a two-foot wide trench.
The Slinky™ Method
The pipe is looped to allow more pipes in a shorter trench, which cuts down on installation costs and makes horizontal installation possible in areas not possible with conventional horizontal applications. Large commercial buildings and schools often use vertical systems because the land area required for horizontal loops would be prohibitive
Vertical
This type of system may be used when the soil is too shallow for trenching or when one does not want to disturb the existing landscaping.
For a vertical system, holes (approximately four inches in diameter) are drilled about 20 feet apart and 100 to 400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building
Pond
If a home has source surface water, such as a pond or lake, this type of loop design may be the most economical, since there is no need to dig a trench or a well for the pipes in the ground. In this type of system, the fluid circulates through polyethylene piping in a body of water, just as it does in the ground loops. The pipe may be coiled in a slinky shape to fit more of it into a given amount of space. This loop is recommended only if the water level never drops below six to eight feet at its lowest level, to assure sufficient heat-transfer capability. Pond loops used in a closed system result in no adverse impacts on the aquatic system.
Open-Loop Systems
This type of system uses well(s) or surface body water as the heat exchange fluid that circulates directly through the GHP system. Once it has circulated through the system, the water returns to the ground through the well, a recharge well, or a surface discharge. This option is obviously practical only where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.
Factors Affecting the Type of GHP Loop
Geothermal heat pumps (GHPs) can be used effectively almost anywhere in the country. However, the specific geological, hydrological, and spatial characteristics of a site determine the best type of ground loop for a specific location.
Instructions: Click on the hot spots in the image below to view the factors affecting the type of GHP Loop.
Benefits of a GHP System
Click on the benefit listed below to find out more information.
Typically, when heating systems or appliances are compared, all the costs that are incurred—purchase, installation, operation, and maintenance costs—can be combined into a life-cycle cost, the cost of ownership over a period of years. The table below compares the various types of central heating systems:
| Compare | Safety | Installation Cost | Operating Cost | Maintenance Cost | Life-Cycle Cost |
|---|---|---|---|---|---|
| Combustion-Based | A Concern | Moderate | Moderate | High | Moderate |
| Heat Pump | Excellent | Moderate | Moderate | Moderate | Moderate |
| Geothermal or Ground-Source Heat Pump | Excellent | High | Low | Low | Low |
Installation and Operating Costs of GHP Systems
On average, a geothermal heat pump (GHP) system costs about \$2,500 to \$3,500 per ton of capacity, or roughly \$7,500 to 10,000 for a 3-ton unit (typical residential size). In comparison, other systems would cost about \$4,000 with air conditioning.
When included in the mortgage, the homeowner has a positive cash flow from the beginning. For example, say that the extra \$3,500 will add \$30 per month to each mortgage payment. A 3,000 square-foot house in Oklahoma City (see below) has a verified average electric bill of \$60 per month, using a geothermal heat pump. This represents significant savings.
A system using horizontal ground loops will generally cost less than a system with vertical loops.
Geothermal heat pump installations in both new and existing homes can reduce energy consumption 25 to 75 percent compared to older or conventional replacement systems. Annual operating costs were also lowest with geothermal heat pumps. Add in the benefits of the desuperheater for hot water savings, and it's easy to see how a GHP system is the most efficient available.
Solar Energy for Home Heating
Energy that is received on the roof of a house is more than enough to supply the heating needs of the home. The energy reaching the earth from the sun ranges from 600 to 2000 BTUs per square foot per day (averaged over a year). It is a function of the latitude of the place. The amount of solar radiation reaching the earth is called the insolation. This is a short for incident solar radiation per day.
The earth revolves around the sun with its axis tilted toward the plane of rotation.
- In June, the North Pole is tilted toward the sun and the solar rays are incident perpendicularly. Therefore, the sun appears to be at a higher angle.
- In December, the North Pole is tilted away from the sun and therefore days are shorter and the solar rays are incident more obliquely, with lower energy flux (winter).
The sun also changes in position and angle from the earth during various times of the year.
Instructions: Press play to observe the movement of the sun as a function of the seasons.
Active Solar Heating Systems
Solar heating systems are classified as “active” or “passive” solar heating systems, or a combination of both. We will first look at active systems.
Active solar heating systems are comprised of collectors, a distribution system, and a storage device.
Instructions: Click on the hot spots in the image below to find out more about the main components of an active solar heating system.
Active solar heating systems operate as follows:
- Flat plate collectors are usually placed on the roof or ground in the sunlight. The top or sunny side has a glass or plastic cover to let the solar energy in. The inside space is a black (absorbing) material to maximize the absorption of the solar energy.
- Cold water is drawn from the storage tank by pump #1 and is pumped through the flat plate collector mounted on the roof of the house.
- The water absorbs the solar energy and is returned back to the tank.
- Warm water from the tank is pumped by pump #2 though the heating coil.
- The fan blows air (from the room) over the heated coil, and the heated air then passes into the room and heats the room.
- Cold air sinks to the bottom and is recirculated over the heating coil.
Note: The standby electric coil is automatically turned on and provides the heat when the water temperature to the heating coil drops because of consecutive cloudy days.
Instruction : Click the “play” button to observe the operation of an Active Solar Heating System:
Collector’s Efficiency is the ratio of solar radiation [4] that is captured and transferred to the collector or heat transfer fluid.
The efficiency of a collector can be expressed as:
Typical collector efficiencies range from 50–70 percent.
Passive Solar Heating Systems
Passive systems do not use mechanical devices such as fans, blowers, or pumps to distribute solar heat from a collector. Instead, they take advantage of natural heat flow to distribute warmth. An example of a passive system for space heating is a sunspace or solar greenhouse.
Passive systems also make use of materials with large heat capacities (stone, water, or concrete) to store and deliver heat. These are called thermal masses.
Instructions: Click on the hot spots in the image below to see the essential elements of a passive solar system.
Passive systems can be categorized into three types:
- Direct Gain - Allows the solar energy to come in through the south-facing window panes.
- Indirect Gain - Allows the solar radiation to heat a wall and then the energy is slowly delivered into the interior of the house. Openings in the wall (called a Trombe Wall), as shown in the figure below, promote convective currents:
- Cold room air enters the space between the glass panel and the wall through the bottom opening.
- As this cold air gets heated, it rises to the top and comes in through the top opening.
- Greenhouse Addition - An attached sunspace and/or solar greenhouse heated by the solar energy - where some of the energy is used to grow the plants and some of it is used to heat the interior of the house.
These systems are shown below.
Costs and Benefits
It is usually most economical to design an active system to provide 40 to 80 percent of the home’s heating needs. Systems providing less than 40 percent of the heat needed for a home are rarely cost-effective, except when using solar air heater collectors that heat one or two rooms and require no heat storage.
A well-designed and insulated home that incorporates passive solar heating techniques will require a smaller and less costly heating system of any type, and may need very little supplemental heat other than solar.
The cost of an active solar heating system will vary. A simple window air heater collector can be made for a few hundred dollars. Commercial systems range from $30 to $80 per square foot of collector area, installed. Usually, the larger the system, the less it costs per unit of collector area. Commercially available collectors come with warranties of 10 years or more, and should easily last decades longer.
Heating your home with an active solar energy system can significantly reduce your fuel bills in the winter. A solar heating system will also reduce the amount of air pollution and greenhouse gases.
Environmental Protection
Ways to Improve Energy Efficiency of Heating Systems
Instructions: Click on the hot spots in the image below to find out how you can improve the energy efficiency of a heating system.