Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
A new method of defrosting evaporator coils
Danfoss A/S, Thermodynamics and Product Concepts, Nordborgvej 81, 6430 Nordborg, Denmark .
Danfoss A/S, Thermodynamics and Product Concepts, Nordborgvej 81, 6430 Nordborg, Denmark .
2012 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 39, 78-85 p.Article in journal (Refereed) Published
Abstract [en]

A new method is presented to defrost evaporator coils of heat pumps using air as a heat source. At low outdoor temperatures the evaporation temperature can drop below the freezing point of water, the water vapor in the air then freezes on the outer surface of the coil. This increases air side pressure drop and reduces the heat transfer capability of the evaporator coil, leading to a decrease in system efficiency. Long frost build-up times would lead to a partly or totally blocked evaporator coil, rendering the system inoperable. To maintain the functionality of the system it is therefore necessary to remove the frost regularly. For a reversible air conditioning system this is typically done by reversing the flow of the system. In the reversed mode the outdoor coil serves as a condenser, hereby melting the frost on the coil surface. Each of these defrost cycles however further reduces the system efficiency substantially. The new method uses an actively distributing valve which is able to feed parallel evaporator passes individually. With this valve single evaporator circuits are regularly shut off. While no refrigerant is evaporated in a closed circuit, the coil surface temperature increases and the flow of the ambient air is sufficient to defrost this part of the evaporator as long as the air temperature is above 0 °C. Experimental results show that under standard frost conditions the evaporator can be kept frost-free and even under severe conditions most of the highly inefficient system reversals can be avoided. Thereby system efficiency is increased significantly.

Place, publisher, year, edition, pages
2012. Vol. 39, 78-85 p.
Keyword [en]
Defrost; Energy efficiency, Expansion valve, Heat pump, Microchannel heat exchanger
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-162102DOI: 10.1016/j.applthermaleng.2012.01.033ISI: 000302670200011Scopus ID: 2-s2.0-84856618078OAI: oai:DiVA.org:kth-162102DiVA: diva2:796904
Note

QC 20150324

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Economic analysis of air-water heat pump technologies with a screening method
Open this publication in new window or tab >>Economic analysis of air-water heat pump technologies with a screening method
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Early on in the process of product development a decision has to be made which technologies to focus on. Under a purely techno-economic viewpoint for a heat pump only these technologies should be considered which maximize heat pump performance for a given cost. Finding these optima is in practice far from trivial as the result is inuenced by variations in operating conditions, interactions between components, model assumptions and uncertain economic data. The overall objective of this thesis is therefore to develop a screening method for the evaluation and comparison of different technologies in regard of cost and efficiency with the aim to identify optimal heat pump designs.

In practice the applicability of such a method depends on the required effort.  Simple mathematical models and short computation times are as mandatory as reliable, coherent and sufficiently general results. To this end the screening method combines methods of simulation, annual performance calculation, metamodeling and optimization.

The screening method is employed for air-water heat pumps in three utilization examples. In the first example the operational costs of maldistribution in evaporators are quantified. It is shown that for air-water heat pumps increasing the evaporator size is no liable option to counteract maldistribution effects. Two alternative technologies, an adjustment of the cycle layout and a change of the superheat control method, are evaluated under the aspect of total cost of ownership. Only with the second technology noteworthy savings can be achieved compared to the baseline. In the second utilization example on/o_ and variable speed control for regulation of the heat pump capacity are considered. Only for colder climates variable speed control pays o_ for the end consumer in a reasonable time. The study shows the comparison of the two control methods to strongly depend on compressor size. In the third example four different cycle layouts are compared. It is demonstrated that the result of the evaluation depends strongly on practical operating limits and on compressor characteristics. The most promising option is a staged layout with economizer heat exchanger.

Two additional studies consider the modeling of operating conditions in more detail. In the first study the simple approach for calculating annual performance used in the screening method is compared with a comprehensive dynamic model. Optimization for both model approaches results in similar component sizes. In the second study the evaporator model is extended to include a dynamic frost growth model which is used to assess operational costs induced by frosting and defrosting of the evaporator coil. A method to reduce the number of defrosts without negatively affecting heat pump capacity is presented and its feasibility demonstrated with experiments.

The screening method can be extended to include more optimization parameters than used in the presented examples. It can also be applied for other small scale vapor compression systems or it can be integrated in a comprehensive evolutionary optimization procedure to reduce computation time and increase numeric stability. Thus the screening method can be a valuable tool in the product development process for shortening the times until new technologies which safe energy and thereby reduce greenhouse gases are available on the market.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xii, 126 p.
Series
TRITA-REFR, ISSN 1102-0245 ; 15:02
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-162104 (URN)978-91-7595-486-8 (ISBN)
Public defence
2015-04-10, Sal Q2, Osquldasväg 10, KTH, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20150324

Available from: 2015-03-24 Created: 2015-03-20 Last updated: 2015-03-24Bibliographically approved

Open Access in DiVA

No full text

Other links

Publisher's full textScopus

Search in DiVA

By author/editor
Mader, Gunda
In the same journal
Applied Thermal Engineering
Energy Engineering

Search outside of DiVA

GoogleGoogle Scholar

doi
urn-nbn

Altmetric score

doi
urn-nbn
Total: 153 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf