Heat Pump

Heat Pump as a System:

Class Definition of a System - "A group of elements working together directly or indirectly to function as a whole for a specific purpose."

http://www.nbbd.com/tac/lennox/pumpcutaway.jpg

A Heat Pump’s Specific Purpose is to function as a Cooling and/or Heating System.

    Components & Subsystems

1.     Compressor

2.     Fan

3.     Copper Tubing

4.     High Pressure Safety Switch

5.     Reversing Valve

6.     Defrost Control

7.     Protective Wire Mesh

To view a complete description of HOW IT WORKS click here.

COMMON APPLICATIONS FOR HEAT PUMPS

1.  Primary Cooling System

Heat pumps can be used to cool buildings working by themselves, without any other additional cooling systems (VAV, CAV, etc.) assistance.  This type of system that performs its duties without “aid” can be called a unitary system.  A heat pump is a very efficient cooling system; however its heating effectiveness is not as good.

2.  Primary Heating System

By simply reversing the direction of refrigerant flow in a heat pump system, the unit changes it capabilities from cooling a space to heating the same.  The control valve in theory seems relatively simple in design, however is quite complex in application and is the most common failure of all the heat pumps components.  Above approximately 35⁰F, the system seems to work effectively, but under this threshold the outdoor coil begins to experience frosting and the Heat Pump becomes relatively ineffective.  In this case a secondary heating source is usually installed to supply the additional heat to the space.  One example of a secondary heating system commonly used is an electric resistance heater.

3.  Replacement of Primary Systems

Due to the capability to perform both functions of heating and cooling, the traditional boiler and/or chiller aren’t needed, drastically diminishing the cost of the system.  As well as, increasing energy efficiency and saving the building owner money.

 4.  Heat Pump as a Closed Loop System

Some applications call for the heating of the North side of a building while cooling the East in the morning hours for certain loading conditions.  In this case, the “warmed” refrigerant can be piped from the zone requiring the cooling around to the zone requiring “cooling”,  this allows the system to operate very efficiently only “wasting” thermal energy through the piping.

Other Possible System Applications

                window system                                        ducted system                             split and cassette systems

Open-Loop System

An open loop, ground-water heat pump, uses a surface or underground water source (such as a lake, river, or well) as the heat source and sink. Well water designs are the most common and seem to be the most cost effective. The well supplies both domestic water and water for the heat pump.

Approximately 1.5 to 3 gallons per minute of well water are needed per ton of cooling capacity. A 3000 square foot, well-insulated home would typically require 8 to 15 gallons per minute.

Closed-Loop System

The closed-loop ground-coupled system uses a buried or submerged geothermal heat exchanger. This heat exchanger can reject or draw heat from a source such as the earth, a lake, or a pond, by circulating water through a loop.

http://cipco.apogee.net/geo/home.asp

SYSTEM LIMITATIONS

 As previously discussed, the major limitation to a heat pump system occurs during its application as a heat source.  If the ambient outdoor temperature drops below approximately 35 degrees F, the outdoor air coils (Evaporator) begin to “ice up”.  This creates a thermal insulation of ice on the coils and the system is rendered ineffective.

Another limitation to a heat pump system falls under the criteria of ventilation.  In layman’s terms “fresh air” that is needed to be introduced into the indoor environment to promote hospitable conditions for the inhabitants.  For example, if there is poor ventilation the content of CO₂, CO and O₂ could become too high or too low, respectively and produce a hazardous condition.  Therefore, a secondary system is usually implemented to increase the outdoor air being circulated into the space, resulting in an acceptable environment.

NUMERIC PARAMETERS 

Below you will find many numeric parameters and illustrations of heat pump systems.  If you wish to learn more about heat pumps I suggest you visit the websites footnoted and the “Professionals” may be able to explain the system more effectively for you.

 All the following are specifications from units from Trane (C)

This is the standard size for a home heat pump:

As we can see the main difference in each of these small units is in the total amount of BTU's and tons in the output.  They all run with the same SEER (seasonal energy efficiency ratio) but then pending on the different size are able to put out more BTU's.

Typical Heat Pump Factory Specifications

http://www.warmwater.com/pumps.htm 

The previous chart has been compiled by previous terms’ colleagues, http://www.pages.drexel.edu/~lan24/AED1/Assignment%205/news.htm#2.

ENERGY EFFICIENCY

Estimated Savings and Market Potential

Figure 4 shows four breakeven electricity cost curves for ambient air residential water heaters. Each curve shows the average electricity cost (including demand) necessary for a heat pump water heater to be life-cycle cost-effective as compared with an existing electric resistance water heater. The horizontal axis defines the climate location as a function of cooling degree days to a 65°F base temperature (CDD65). The upper two curves assume that space heating is provided by electric resistance heat and the lower two curves assume that space heat is provided by a heat pump. For electric energy costs higher than the chosen curve at a specific CDD65 value, the HPWH is likely to prove cost-effective.

Fig. 4. Breakeven Electricity Cost Curves for an Ambient Air Heat Pump Water Heater Retrofit

The curves in Figure 4 were arrived at using an analysis technique described later in this Alert. To develop them, 32 different locations in the country were analyzed using this technique and the results curve-fitted to the CDD65 value for each site. Assumptions include a 6,000 Btu/hr ambient air HPWH retrofit with an installed cost of $985 and air-conditioning in the residence (nominal SEER 9.0). A daily average COP of 2.5 was assumed for this analysis. Differing first costs, family sizes, changes in air-conditioning use, and use of exhaust air HPWHs would generate different results.

Visit http://www.pnl.gov/fta/3_res.htm for more information on heat pump efficiency.