Pros, Cons, and Opinions about Heat Pumps

The general consensus about geothermal energy pumps, is nearly overwhelmingly positive. They represent the most energy-efficient and environmentally clean heating and cooling systems available. The US Department of Energy said that heat pumps can save a typical home hundreds of dollars each year, and pay itself off in around 8-12 years. Since 2008 tax credits have been available for Energy Star systems that can reduce this payback period. Geothermal systems can be especially useful in rural areas, where propane or electricity would normally be used for heating and cooling, but heat pumps are considerably less expensive. Geothermal energy is very up and coming, with great potential as a long-term sustainable energy system.

There are some downsides to geothermal heat pumps. The most considerable of which is cost. Although heat pumps have a relatively short payback period, and long-term savings, heat pumps are more expensive at installation than a typical heating and cooling system. Additionally, they are possibly not the best option for existing structures, because installation requires digging up the areas surrounding the building. Due to its relative ‘newness’ as an energy source, many heating and cooling installers may not be familiar with the technology.

  • Ashley’s Opinion :
    • In my opinion, assuming the technology is available to you in your area, geothermal heat pumps seem like the best option for anyone’s heating and cooling system. Environmentally, the systems are so efficient and so quickly return any initial investment, that cost would be a moot point. In the building of a new home, one would just add the additional expense of the geothermal heating system to their mortgage, and in the grand scheme would make the expense nearly negligible. However, if I were living in an old home, I would not go out of my way to replace my heating system with a geothermal one, the digging would be extensive and have higher upfront cost.
  • Wanessa’s Opinion :
    • For me, heat pumps are one great idea that is good for the environment and for your pocket. People may think that because it cost a lot of money to install, it is not good. However, the money that you invested in heats pumps will be back in your pocket in a few years. It was proved that heat pumps are really efficient, and even though you have an old house I believe that would be good to install heat pumps,at least, I would. Heat pumps are the future, and the future is already here.
  • Emily’s Opinion :
    • Although installing a geothermal heat pump is expensive (generally ranging from $10,000 to $30,000), the benefits outway its cost.  The geothermal heat pump has a much lower operating cost than other systems and will use clean and renewable energy!  Another benefit is it is quiet and the geothermal energy pump can last about fifty years.  In my opinion the geothermal heat pump will pay for itself and save you money.

 

References

  • Web. 11 Dec. 2015. <http://www.climatemaster.com/downloads/RP215.pdf>.
  • “How Geothermal Energy Works.” Union of Concerned Scientists. 22 Dec. 2014. Web. 11 Dec. 2015.
  • “Geothermal Heat Pumps.” Geothermal Heat Pumps. US Department of Energy. Web. 11 Dec. 2015.

Thermodynamic Principles of a Heat Pump

Conversion of heat into work is difficult and is limited by the laws of thermodynamics. A part of the heat used has to be rejected to the surroundings, so there is always an upper limit of the possible work production from a given heat stream.

The second law of thermodynamics demands that a part of the heat input to any heat engine is rejected to the environment. The portion of the input heat, which can be converted into work, is called exergy (availability, convertible energy). The unconvertible portion is called anergy. Thus the exergy of any system is equal to the maximum work (or which can be produced from the source

 

Figure 1. Heat Engine showing the Kelvin  Statement of the 2nd Law of Thermodynamics

Figure 1. Heat Engine showing the Kelvin Statement of the 2nd Law of Thermodynamics

Heat pump cycles are similar to heat engine cycles but the work in reverse and are known as reversed heat engine cycles. Heat is withdrawn from the cold reservoir (surroundings) and deposited into the hot reservoir. Work must be done on the system to effect this direction of heat flow because it is not a spontaneous process.

Figure 2. Simple Diagram of a Heat Pump

Figure 2. Simple Diagram of a Heat Pump

The coefficient of performance: the ration of the heat pumped into the hot reservoir to the work input to the heat pump. The heat pump becomes less effective as Tcold decreases.

 

References:

Engel T, Reid P. 2013. Thermodynamics, Statistical Thermodynamics, and Kinetics. Third. USA: Pearson.

Valdimarsson P. 2011 Jan 16. Thermodynamics of Geothermal Power Production [review]. LaGeo [accessed 2015 Dec 2]. http://www.os.is/gogn/unu-gtp-sc/UNU-GTP-SC-12-34.pdf.

Best Efficiency Based on Thermodynamics

A heat pump does not “create” heat, simply moves it. The heat pump applies external work to extract an amount of heat from a cold place and delivers heat to a hot place. A heat pump has the same limitations from the second law of thermodynamics as any other heat engine and therefore a maximum efficiency can be calculated from the Carnot Cycle.

Heat pumps are more effective for heating than for cooling an interior space if the temperature differential is held equal. Due to Carnot efficiency limits, the heat pump’s performance will decrease as the outdoor-to-indoor temperature difference increases. As one part of the heat pump is outside, if the temperature is too cold that froze the outdoor part, the outdoor part will melt the ice to work and this defrosting use an additional energy.

When talking about heat pumps it is better to avoid the word “efficiency”. For Heat pumps is more used the term coefficient of performance (COP) that describe the ratio of useful heat movement per work input. The work can come for various sources.

“According to the US EPA, geothermal heat pumps can reduce energy consumption up to 44% compared with air-source heat pumps and up to 72% compared with electric resistance heating.” (wikipedia.org, 2015)

For example, if comparing a typical air source heat pump that has a COP of 3 to 4 to an electrical resistance heater that has a COP of 1, in a day with 10°C of temperature, the electrical heater will produce only one joule of useful heat with one joule of electrical energy, while one joule of electrical energy of the heat pumps will produce 3 to 4 joules.

The coefficient of performance (COP) or (CP) is:

cp1

There is a theoretical maximum CP of the Carnot Cycle :

cp2

Where, Qc is the heat extracted, and QH the heat exhausted.

For a refrigerator, the coefficient of performance of a refrigerator is expressed as:

cp3

Energy flows (Carnot cycle):

cp4

As the illustration shows, an electric heat pump can deliver more heat to a house than burning the primary fuel at 100% efficiency inside the house. That is higher efficiency than a typical forced-air natural gas furnace which brings the primary fuel to the house.

References:

https://en.wikipedia.org/wiki/Heat_pump#Efficiency

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatpump.html#c2

http://physics.ucsd.edu/do-the-math/2012/06/heat-pumps-work-miracles/

http://www.uccs.edu/~tchriste/courses/PES100/100lectures/heatpumps.html

Environmental Impacts of Geothermal Heat Pumps

 

  • Plants vs Residential/Commercial heat pump systems
    • Geothermal plants
      • Generally use closed loop systems
        • these do not open to the air, no harmful emissions are produced
        • extracted water goes directly back into the geothermal reservoir after being used to generate heat
      • Some use open loop systems
        • These can emit hydrogen sulfide, carbon dioxide, ammonia, methane, boron, and nitrous oxides
          • hydrogen sulfide is the most common emission 
          • hydrogen sulfide (H2S) converts to sulfur dioxide (SO2) in the atmosphere
          • Nitrous oxides and sulfur dioxide producing acid rain (dissolved acids) upon oxidation by ozone (O3)
      •  Location
        • the most developed type of geothermal heat plants are located on geologic ‘hot spots’
          • these areas are already unstable
          • there is evidence that a geothermal plant can increase earthquake frequency in those regions
        • Enhanced geothermal systems, wherein water is pumped underground into hot rock reservoirs, in order to fracture the rock, can increase the risk of small earthquakes
      • Geothermal plants account for more than 25 percent of the electricity produced in both Iceland and El Salvador
    • Residential/ Commercial
      • Open loop
        • once water circulates through system, it returns to a recharge well or by surface discharge
          • unless codes are met, surface discharge may be harmful by introducing unclean water into the area
      • Closed loop
        • can be horizontal, vertical, slinky, or pond/lake types of geothermal pumps
          • these may disturb wildlife to some extent due to the extensive digging needed for the wells or loops

References

  • Russel, Randy. Sulfur trioxide. 2006. Windows To The Universe. Web. 8 December 2015.
  • Pidwirny, M. (2006). “Acid Precipitation”. Fundamentals of Physical Geography, 2nd Edition. April 11, 2012. <http://www.physicalgeography.net/fundamentals/8h.html>
  • “The Geothermal City: Technologies.” The Geothermal City: Technologies. Web. 9 Dec. 2015.
  • “How Geothermal Energy Works.” Union of Concerned Scientists. 22 Dec. 2014. Web. 11 Dec. 2015.

Engineering

Geothermal heat pumps use the constant temperature of the ground (10-14°C) to heat and cool structures. The pumps use a system of buried pipes or “loops” that use a refrigeration cycle, much like a refrigerator, to transfer heat. For example, the heat from the ground is absorbed as a fluid at low temperature inside the loop. The fluid then passes through a compressor that raises it to a higher temperature, which can then heat water and the rooms of the structure. In the cooled ground-loop system fluid passes back into the ground where it absorbs further energy in a continuous process.

4-1-5-geoheat ghp
There are two different types of geothermal looped systems, open and closed. In an open system groundwater from a well is directly used as an energy source. In the closed loop system there is four different kinds of loops. First a horizontal loop is where pipes are placed in trenches that range 100 to 400 feet in length and 3 to 6 feet in depth. This method is generally preferred when the building has plenty of space. When space is limited vertical loops are used. Well drilling equipment creates small-diameter holes 100 to 400 deep. If a body of water is readily available it can be used for a pond loop. The coils of the pipe are placed on the bottom of the pond or lake to capture the geothermal energy. Lastly, the slinky coil is a flattened, overlapped circular coiled ground loop heat exchanger. It concentrates the heat transfer surface into a smaller volume, requiring less land area and shorter trenching.

geo_types

References:

“Geothermal – Slinky Coils.” Geothermal – Slinky Coils. N.p., n.d. Web. 01 Dec. 2015.
“Geothermal Heating & Cooling – How Does Geothermal Energy Work?” ClimateMaster. N.p., n.d. Web.
“Ground Source Heat Pumps.” Ground Source Heat Pumps. N.p., n.d. Web. 01 Dec. 2015.
“Harnessing Geothermal Energy.” Geothermal Energy. N.p., n.d. Web. 01 Dec. 2015.
“How Do Geothermal Heat Pumps Work?” How Do Geothermal Heat Pumps Work? N.p., n.d. Web. 01 Dec. 2015.