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Economic analysis of air-water heat pump technologies with a screening method
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
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: urn:nbn:se:kth:diva-162104ISBN: 978-91-7595-486-8 (print)OAI: oai:DiVA.org:kth-162104DiVA: diva2:796922
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
List of papers
1. Maldistribution in air-water heat pump evaporators: Part 1: Effects on evaporator, heat pump and system level
Open this publication in new window or tab >>Maldistribution in air-water heat pump evaporators: Part 1: Effects on evaporator, heat pump and system level
2015 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 50, 207-216 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents an approach to quantify the effect of evaporator maldistribution on operating costs of air-water heat pumps. In the proposed simulation model maldistribution is induced by two parameters describing refrigerant phase and air flow distribution. Annual operating costs are calculated based on heat pump performance at distinct operating conditions. Results show that percentage increase of operating costs is similar for the three considered climate zones, even though the effect of maldistribution on heat pump performance varies with operating conditions. Differences in terms of absolute cost increase for the climate zones arise mainly due to a varying number of operating hours. Absolute cost increase is considerable in the average and especially colder climate zone and can only partly be reduced by enlarging the evaporator.

Keyword
Annual performance, Heat pump, Maldistribution, Operating costs, System modeling
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-162095 (URN)10.1016/j.ijrefrig.2014.07.006 (DOI)000350091900021 ()2-s2.0-84922267606 (Scopus ID)
Note

QC 20150324

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2017-12-04Bibliographically approved
2. Maldistribution in air-water heat pump evaporators. Part 2: Economic analysis of counteracting technologies
Open this publication in new window or tab >>Maldistribution in air-water heat pump evaporators. Part 2: Economic analysis of counteracting technologies
2015 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 50, 217-226 p.Article in journal (Refereed) Published
Abstract [en]

In this study a methodology is applied to quantify the effect of evaporator maldistribution on operating costs of air-water heat pumps. The approach is used to investigate the cost-effectiveness of two technologies enabling to counteract maldistribution: a flash gas bypass setup and the individual superheat control in parallel evaporator channels. In the total cost of ownership analysis, different scenarios for climatic conditions, severity of maldistribution, and economic framework are considered. Results show that the flash gas bypass system is cost-effective only in a few conditions, namely severe maldistribution, high electricity prices, and colder climate. Investment in the individual superheat control technology, however, can be quickly amortized in many scenarios. For the warmer climate zone with a small number of operating hours counteracting of maldistribution does not pay off under the used economic assumptions.

Keyword
Annual performance, Heat pump, Maldistribution, Optimization, Total cost of ownership
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-162096 (URN)10.1016/j.ijrefrig.2014.07.005 (DOI)000350091900022 ()2-s2.0-84922242641 (Scopus ID)
Note

QC 20150327

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2017-12-04Bibliographically approved
3. Capacity control in air-water heat pumps: Total cost of ownership analysis
Open this publication in new window or tab >>Capacity control in air-water heat pumps: Total cost of ownership analysis
2014 (English)In: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 81, 296-304 p.Article in journal (Refereed) Published
Abstract [en]

Adjusting capacity to changing demand by variable speed control is known to offer efficiency improvement over classical on/off control. With a total cost of ownership analysis the economic viability of both control schemes is assessed for residential air-water heat pumps operating in different climate zones. Component sizes are optimized for both control methods individually. Results show optimal compressor displacement volumes to be smaller for variable speed than for on/off control. The optimal ratio of evaporator to condenser size is smaller for the variable speed system. Variable speed control is shown to be uneconomic for space heating in warmer climate while for average climate cost-effectiveness depends on the economic framework. For colder climate variable speed control is the more profitable choice in all considered cases; savings of up to 5000 EUR compared to on/off control can be achieved within 15 years of operation.

Keyword
Heat pump, Capacity control, Seasonal performance, Optimization, Economics, System modeling
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-156119 (URN)10.1016/j.enbuild.2014.06.029 (DOI)000343363700029 ()2-s2.0-84904322055 (Scopus ID)
Note

QC 20150108

Available from: 2015-01-08 Created: 2014-11-21 Last updated: 2017-12-05Bibliographically approved
4. Economic potential of cycle layout changes in residential R290 air-water heat pumps.
Open this publication in new window or tab >>Economic potential of cycle layout changes in residential R290 air-water heat pumps.
(English)Manuscript (preprint) (Other academic)
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-162098 (URN)
Note

QS 2015

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2015-03-24Bibliographically approved
5. Concepts to increase the seasonal coefficient of performance of an air-water heat pump: Comparison of costs and benets
Open this publication in new window or tab >>Concepts to increase the seasonal coefficient of performance of an air-water heat pump: Comparison of costs and benets
2013 (English)Conference paper, Published paper (Refereed)
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-162099 (URN)
Conference
Clima2013 - 11th REHVA World Congress, Prague, Czech Republic
Note

QC 20150324

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2015-03-24Bibliographically approved
6. Economic optimization of component sizes in a ground source heat pump system
Open this publication in new window or tab >>Economic optimization of component sizes in a ground source heat pump system
(English)Manuscript (preprint) (Other academic)
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-162101 (URN)
Note

QS 2015

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2015-03-24Bibliographically approved
7. A new method of defrosting evaporator coils
Open this publication in new window or tab >>A new method of defrosting evaporator coils
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.

Keyword
Defrost; Energy efficiency, Expansion valve, Heat pump, Microchannel heat exchanger
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-162102 (URN)10.1016/j.applthermaleng.2012.01.033 (DOI)000302670200011 ()2-s2.0-84856618078 (Scopus ID)
Note

QC 20150324

Available from: 2015-03-20 Created: 2015-03-20 Last updated: 2017-12-04Bibliographically approved

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