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Combined solar and pellet heating systems for single-family houses: How to achieve decreased electricity usage, increased system efficiency and increased solar gains
Dalarna University, School of Technology and Business Studies, Environmental Engineering. KTH, Skolan för industriell teknik och management (ITM), Energiteknik.
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In Sweden, there are about 0.5 million single-family houses that are heated by electricity alone, and rising electricity costs force the conversion to other heating sources such as heat pumps and wood pellet heating systems. Pellet heating systems for single-family houses are currently a strongly growing market. Future lack of wood fuels is possible even in Sweden, and combining wood pellet heating with solar heating will help to save the bio-fuel resources. The objectives of this thesis are to investigate how the electrically heated single-family houses can be converted to pellet and solar heating systems, and how the annual efficiency and solar gains can be increased in such systems. The possible reduction of CO-emissions by combining pellet heating with solar heating has also been investigated. Systems with pellet stoves (both with and without a water jacket), pellet boilers and solar heating have been simulated. Different system concepts have been compared in order to investigate the most promising solutions. Modifications in system design and control strategies have been carried out in order to increase the system efficiency and the solar gains. Possibilities for increasing the solar gains have been limited to investigation of DHW-units for hot water production and the use of hot water for heating of dishwashers and washing machines via a heat exchanger instead of electricity (heat-fed appliances). Computer models of pellet stoves, boilers, DHW-units and heat-fed appliances have been developed and the parameters for the models have been identified from measurements on real components. The conformity between the models and the measurements has been checked. The systems with wood pellet stoves have been simulated in three different multi-zone buildings, simulated in detail with heat distribution through door openings between the zones. For the other simulations, either a single-zone house model or a load file has been used. Simulations were carried out for Stockholm, Sweden, but for the simulations with heat-fed machines also for Miami, USA. The foremost result of this thesis is the increased understanding of the dynamic operation of combined pellet and solar heating systems for single-family houses. The results show that electricity savings and annual system efficiency is strongly affected by the system design and the control strategy. Large reductions in pellet consumption are possible by combining pellet boilers with solar heating (a reduction larger than the solar gains if the system is properly designed). In addition, large reductions in carbon monoxide emissions are possible. To achieve these reductions it is required that the hot water production and the connection of the radiator circuit is moved to a well insulated, solar heated buffer store so that the boiler can be turned off during the periods when the solar collectors cover the heating demand. The amount of electricity replaced using systems with pellet stoves is very dependant on the house plan, the system design, if internal doors are open or closed and the comfort requirements. Proper system design and control strategies are crucial to obtain high electricity savings and high comfort with pellet stove systems. The investigated technologies for increasing the solar gains (DHW-units and heat-fed appliances) significantly increase the solar gains, but for the heat-fed appliances the market introduction is difficult due to the limited financial savings and the need for a new heat distribution system. The applications closest to market introduction could be for communal laundries and for use in sunny climates where the dominating part of the heat can be covered by solar heating. The DHW-unit is economical but competes with the internal finned-tube heat exchanger which is the totally dominating technology for hot water preparation in solar combisystems for single-family houses.

Place, publisher, year, edition, pages
Stockholm: Department of Energy and Environmental Technology, KTH - Royal Institute of Technology , 2006.
Keyword [en]
Pellet, pelleteldning, kamin, panna, solvärmesystem, varmvattenberedning, systemutformning, småhus, elbesparing, verkningsgrad, emissioner, rökgasförluster, skorstensförlust, diskmaskin, tvättmaskin
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:du-2405OAI: oai:dalea.du.se:2405DiVA: diva2:523356
Public defence
2006-12-13, Sal M3, KTH, Brinellvägen 64, Stockholm, 10:00
Opponent
Supervisors
Available from: 2006-12-05 Created: 2006-12-05 Last updated: 2013-09-03Bibliographically approved
List of papers
1. Computer modelling of wood pellet stoves and boilers connected to solar heating systems
Open this publication in new window or tab >>Computer modelling of wood pellet stoves and boilers connected to solar heating systems
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2006 (English)In: Proc on USB of Pellets 2006, 30 May - 1 June, Jönköping, Sweden, Jönköping, Sweden, 2006Conference paper, (Other academic)
Abstract [en]

When optimizing systems for wood pellet and solar heating, there is a need for realistic computer models of stoves and boilers in order to perform simulation studies. The objective of this work was to develop and verify a mathematical model for wood pellet stoves and boilers for use in system simulations with TRNSYS calculating both the energy balance and CO-emissions (carbon monoxide). Laboratory measurements have been carried out on three pellet stoves, one traditional and two with gas-liquid heat exchangers, and four pellet boilers. A mathematical two-node model of a stove was developed and implemented as a TRNSYS component. Parameters were identified for two stoves and three boilers. This new model makes it possible to perform detailed simulations with time steps less than a minute of complete wood pellet heating systems and to derive long term values, such as annual values, of efficiency and emissions for the boiler or stove in a system context under realistic conditions. In addition, parametric studies can be used in order to investigate how different operation principles and system design affect these values. The simulated energy balance of a water jacketed stove investigated in this work agreed well with measured data during both stationary and dynamic conditions.

Place, publisher, year, edition, pages
Jönköping, Sweden: , 2006
Keyword
Pellets, stoves, boilers, efficiency, emissions, emission reduction
Identifiers
urn:nbn:se:du-2105 (URN)
Conference
Pellets 2006, Jönköping, Sweden, 30 May - 1 June, 2006
Available from: 2006-05-16 Created: 2006-05-16 Last updated: 2013-09-03Bibliographically approved
2. Electrical savings by use of wood pellet stoves and solar heating systems in electrically heated single-family houses
Open this publication in new window or tab >>Electrical savings by use of wood pellet stoves and solar heating systems in electrically heated single-family houses
2005 (English)In: Energy and Buildings, ISSN 0378-7788, E-ISSN 1872-6178, Vol. 37, no 9, 920-929 p.Article in journal (Refereed) Published
Abstract [en]

This study investigates how electrically heated single-family houses can be converted to wood pellets- and solar heating using pellet stoves and solar heating systems. Four different system concepts are presented and system simulations in TRNSYS evaluate the thermal performance and the electrical savings possible for two different electrically heated single-family houses. Simulations show that the electricity savings using a wood pellet stove are greatly affected by the level of comfort, the house plan, the system choice and if the internal doors are open or closed. Installing a stove with a water-jacket connected to a radiator system and a hot water store has the advantage that heat can be transferred to domestic hot water and be distributed to other rooms. Such systems leads to that more electricity can be replaced, especially in houses having a traditional plan. Though it is unnecessary to have too many radiators connected to a stove with a low fraction of energy heating the water jacket. Today’s most common control strategy for stoves, (the on/off-control) results in unnecessarily high emissions. A more advanced control varying the heating rate from maximum to minimum to keep a constant room temperature reduces the number of starts and stops and thereby the emissions.

Place, publisher, year, edition, pages
Elsevier, 2005
Keyword
wood pellet stove; solar heating system; single-family house
National Category
Civil Engineering
Identifiers
urn:nbn:se:du-730 (URN)10.1016/j.enbuild.2004.10.013 (DOI)000231676000002 ()
External cooperation:
Available from: 2004-12-15 Created: 2004-12-15 Last updated: 2016-08-29Bibliographically approved
3. Thermal performance of combined solar and pellet heating systems
Open this publication in new window or tab >>Thermal performance of combined solar and pellet heating systems
2006 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 31, no 1, 73-88 p.Article in journal (Refereed) Published
Abstract [en]

Various pellet heating systems are marketed in Sweden, some of them in combination with a solar heating system. Several types of pellet heating units are available and can be used for a combined system. This article compares four typical combined solar and pellet heating systems: System 1 and 2 with a pellet stove, system 3 with a store integrated pellet burner and system 4 with a pellet boiler. The often lower efficiency of pellet heaters compared to oil or gas heaters increases the final energy demand. Consequently heat losses of the various systems have been studied. The systems have been modeled in TRNSYS and simulated with parameters identified from measurements. For almost all systems the flue gas losses are the main heat losses except for system 3 where store heat losses prevail. Relevant are also the heat losses of the burner and the boiler to the ambient. Significant leakage losses are noticed for system 3 and 4. For buildings with an open internal design system 1 is the most efficient solution. Other buildings should preferably apply system 2 or 3. The right choice of the system depends also on whether the heater is placed inside or outside of the heated area. Unlike the expectations and results from other studies, the operation of the pellet heaters with modulating combustion power is not necessarily improving the performance. A large potential for system optimization exists for all studied systems, which when applied could alter the relative merits of the different system types.

Keyword
pellet heating systems; heat losses; flue gas losses; leakage losses
National Category
Civil Engineering
Identifiers
urn:nbn:se:du-1045 (URN)10.1016/j.renene.2005.03.007 (DOI)000232741200006 ()
External cooperation:
Available from: 2005-05-10 Created: 2005-05-10 Last updated: 2016-08-29Bibliographically approved
4. Increasing efficiency and decreasing CO-emissions for a combined solar and wood pellet heating system for single-family houses
Open this publication in new window or tab >>Increasing efficiency and decreasing CO-emissions for a combined solar and wood pellet heating system for single-family houses
2006 (English)In: Proc. on USB of Pellets 2006, 30 May - 1 June, Jönköping, Sweden, Jönköping, Sweden, 2006Conference paper, (Other academic)
Abstract [en]

This study focus on how wood pellets and solar heating systems for single-family houses should be designed and controlled to reach high efficiency and to reduce the CO-emissions. A recently developed TRNSYS model was used to simulate the wood pellet boiler. Parameters for the model were identified from laboratory measurements on a boiler. A detailed simulation model of a complete solar combisystem was created and annual simulations were performed. Assuming that all heat losses to the room are waist heat, the results show that the most important factors to achieve high system efficiency are that the boiler and the buffer store should be well insulated. The sensor controlling the boiler should be placed in the store; the pump between the boiler and the store should only be in operation together with the burner and for some time after the burner have stopped to take care of the after burning heat. For boilers with relatively large start and stop CO-emissions modulating power may be an efficient measure to reduce CO-emissions. Especially for boilers using an ON-OFF control, the dominating contribution of CO-emissions may be during the start and stop phases, thus reducing emissions during operation may have little influence on the annual CO-emissions.

Place, publisher, year, edition, pages
Jönköping, Sweden: , 2006
Keyword
Boilers, efficiency, emissions, emission reduction
Identifiers
urn:nbn:se:du-2106 (URN)
Conference
Pellets 2006 , Jönköping, Sweden, 30 May - 1 June, 2006
Available from: 2006-05-16 Created: 2006-05-16 Last updated: 2013-09-03Bibliographically approved
5. External DHW Units for Solar Combisystems
Open this publication in new window or tab >>External DHW Units for Solar Combisystems
2003 (English)In: Solar Energy, Vol. 74, 193-204 p.Article in journal (Refereed) Published
Abstract [en]

This article compares seven different external DHW units, comprising flat plate heat exchanger and flow control, with a reference method for preparing hot water. These DHW units use different control methods. The objective of the study was to determine which methods are most effective in solar combisystems and to identify other factors that strongly influence the energy savings of the system. Five of the DHW units were judged to be of interest for the study because of their measured performance or the simplicity of their design. Of these, measurement data showed that two had the same control function although of different physical construction. Thus four DHW units were modelled in the simulation environment PRESIM/ TRNSYS, parameters were identified from measured data, and annual simulations were performed with a number of parametric variations. Three of the DHW units performed significantly better than the reference system provided that they were sized correctly: microprocessor control with variable-speed pump; proportional controller with regulating valve; and a turbine pump. The most important design factors identified by the study were: the maximum possible primary flow, which needs to be suitable for the design hot water load profile; and ensuring a low temperature is returned to the store. The hot water load profile was also shown to strongly influence the energy savings, assuming that auxiliary heater’s thermostat is set so that the system just meets the worst-case discharge.

Keyword
Solar heating; DHW preparation; Solar combisystem
Identifiers
urn:nbn:se:du-22 (URN)10.1016/S0038-092X(03)00158-0 (DOI)000184888500002 ()
Available from: 2004-05-19 Created: 2004-05-19 Last updated: 2013-09-03Bibliographically approved
6. Dishwasher and washing machine heated by a hot water circulation loop
Open this publication in new window or tab >>Dishwasher and washing machine heated by a hot water circulation loop
2007 (Swedish)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 27, no 1, 120-128 p.Article in journal (Refereed) Published
Abstract [en]

Electric energy (70-90%) used by electrically heated dishwashers and washing machines is used for heating the water, the crockery, the laundry and the machine and could as well be replaced by heat from other sources than electricity. This article evaluates prototypes of a dishwasher and a washing machine, where the machines are heated by a hot water circulation loop and the heat is transferred to the machines via a heat exchanger. The machine therefore uses water from the cold water pipe. Measurements and simulations have been performed showing that all energy for heating can be replaced if the supply water temperature is 65-70 degrees C. An alternative and common way to save electricity is to connect the machines to the domestic hot water pipe, but the electrical savings with this measure are much smaller, especially for the dishwasher. Computer modelling has been performed and the model has proved to have a high agreement with measured data. However comparison with manufacturers' data indicates that the computer models overestimate the energy demand by about 10 %.

Keyword
dishwasher, clothes washer, washing machine, domestic appliances, heat exchanger
National Category
Energy Engineering
Identifiers
urn:nbn:se:du-12893 (URN)10.1016/j.applthermaleng.2006.05.005 (DOI)000241569000014 ()
Available from: 2013-09-03 Created: 2013-09-03 Last updated: 2015-10-19Bibliographically approved
7. Increasing solar gains by using hot water to heat dishwashers and washing machines
Open this publication in new window or tab >>Increasing solar gains by using hot water to heat dishwashers and washing machines
2007 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 27, no 2-3, 646-657 p.Article in journal (Refereed) Published
Abstract [en]

70 to 90% of the electric energy used by dishwashers and washing machines heats the water, the crockery, the laundry and the machine and could just as well be replaced by heating energy from solar collectors, district heating or a boiler. A dishwasher and a washing machine equipped with a heat exchanger and heated by a hot water circulation circuit instead of electricity (heat-fed machines) have been simulated together with solar heating systems for single-family houses in two different climates (Stockholm, Sweden and Miami, USA). The simulations show that a major part of the increased heat load due to heat-fed machines can be covered by solar heat both in hot and cold climates if the collector area is compensated for the larger heat load to give the same marginal contribution. Using ordinary machines connected to the hot water pipe (hot water-fed machines) and using only cold water for the rinses in the washing machine gives almost the same solar contribution; however considerably lower electrical energy savings are achieved. The simulations also indicate that improvements in the system design of a combisystem (increased stratification in the store) are more advantageous if heat-fed machines are connected to the store. Thus, using heat-fed machines also encourages the use of more advanced solar combisystems.

Place, publisher, year, edition, pages
Elsevier, 2007
Keyword
Dishwasher; Clothes washer; Washing machine; Domestic appliances; Solar heating system; Solar combisystem; Solar hot water system; SDHW; System design.
National Category
Civil Engineering
Identifiers
urn:nbn:se:du-2123 (URN)10.1016/j.applthermaleng.2006.05.027 (DOI)000241706500042 ()
Available from: 2006-05-23 Created: 2006-05-23 Last updated: 2015-10-19Bibliographically approved

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