1 Hybrid heating systems and smart grid System design and operation market status Project report April 2013 REPORT Danish Gas Technology Centre Dr. Neergaards Vej 5B DK-2970 Hørsholm Tlf Fax
2 Hybrid heating systems and smart grid System design and operation - market status Mikael Näslund Danish Gas Technology Centre Hørsholm 2013
3 Title : Hybrid heating systems and smart grid Report category : Project report Author : Mikael Näslund Date of issue : Copyright : Dansk Gasteknisk Center a/s File number : ; H:\737\36 Hybridsystemer\Rapport\Hybridrapport final.docx Project name : Hybridsystemer til gas og el i samspil med smart grid ISBN :
4 DGC-report 1 Table of contents Page Summary Introduction Hybrid system definition Early work and appliances Current hybrid system appliances Products on the market Buderus Daalderop Vaillant Junkers/Bosch Viessmann Glow-worm MHG Carrier Summary of hybrid systems Hybrid model and calculations Calculation parameters Gas boiler operates as peak load capacity control Price controlled operation Energy costs for the hybrid system Conclusions References... 34
5 DGC-report 2 Summary In future smart grids, where gas and electricity grids are interacting, appliances that use both gas and electricity are highly interesting. New gas appliances for space heating fit into the smart-grid concept. These appliances operate on the consumer level and do not directly require active involvement from the inhabitants or changes in their daily energy consumption pattern. The gas-based technologies suitable for smart-grid integration are hybrid systems and micro cogeneration. Hybrid systems have the possibility to switch between gas and electricity for the space heating while micro cogeneration produces both heat and electricity. Both technologies offer the possibility to be externally controlled and used for grid balance and thus maximizing the efficient use of renewable electricity. The gas grid acts as support and back-up for intermittent renewable energy with these gas appliances. This study focuses on hybrid systems consisting of a condensing gas boiler and a small air-to-water heat pump. A market survey of product status 2012 is presented and simulations of hybrid system annual performance in Danish installations are presented. The marketed hybrid systems can be divided into three basic designs: gas boiler and heat pump in separate cabinets, gas boiler and heat pump integrated in a common cabinet and heat pump and control systems that can be added to existing gas boilers. The annual performance calculations show that the gas consumption for an annual space heating demand of 20,000 kwh and 2,000 kwh hot water demand will be reduced to the 5,000-10,000 kwh range including hot water production. The exact number depends on heat pump size, COP and energy costs. The cost for gas and electricity for the consumer is likely to be reduced by up to % with the current Danish energy prices and a reasonably efficient and sized hybrid system. The investment and maintenance costs for the consumer are not known today due to the fact that hybrid systems are only at a market introduction stage.
6 DGC-report 3 The advantages for the gas and electricity utilities lie mainly in the possibility of using the hybrid systems as part of a smart grid where gas and electricity interact. Renewable electricity can be used in the best possible way. The hybrid systems can be switched over to gas operation when for example the wind power output is low and the option is expensive power generation with high CO 2 emissions.
7 DGC-report 4 1 Introduction The Smart Grids European Technology Platform 1 defines a smart grid as A Smart Grid is an electricity network that can intelligently integrate the actions of all users connected to it generators, consumers and those that do both in order to efficiently deliver sustainable, economic and secure electricity supplies. A graphical representation of EU smart grid projects is shown in Figure 1 2. This report describes a study that deals with a technology that can be characterized either as home application or integrated system. The map clearly shows that investments in smart meters are by far the most common action. The gas grid is also more and more considered as an important part of the smart grid concept, since using the gas grid is the most efficient way of storing energy. Figure 1 Overview EU smart grid projects 1 2
8 DGC-report 5 New gas appliances for heating include gas and solar energy, gas heat pumps, micro cogeneration and hybrid systems. In this study a hybrid system is defined as a gas boiler and an electric heat pump in an integrated operation. Gas and solar energy and gas heat pumps reduce the gas consumption due to the use of renewable energy. Hybrid systems and micro cogeneration can be described as follows regarding gas consumption. Hybrid systems use less gas energy at the consumer site. Renewable energy from the heat source is used in the heat pump. Electricity may also be renewable. In the future also renewable gas such as upgraded biogas may be distributed in the gas grid. Micro cogeneration increases gas consumption at the consumer site. Grid electricity is replaced by on-site generated electricity. The gain is reduced primary energy consumption since the micro cogeneration is assumed to have a better overall fuel utilization than centralized power plants. Early micro cogeneration appliances use either Stirling engines or small internal combustion engines. The electric efficiency ranges from 10 % to 25 %. In the future fuel cells with higher electric efficiency are supposed to be used. Hybrid systems may be a part of the future smart grid concept where the electricity and gas grids are interacting in order to use renewable electricity as efficiently as possible. In this report a hybrid system for single-family houses is investigated. This report deals with small-scale, downstream heating appliances that may be part of the smart-grid concept. These appliances are described as dualfuel appliances in the EU Commission Task Force for Smart Grids . The following appliances are suggested in the EU document as dual-fuel or dualoutput appliances in a smart gas grid: A heat pump providing heat for base load and a condensing gas boiler for peak loads and often hot water production as well Condensing gas boiler with electrically heated storage tank Micro cogeneration
9 DGC-report 6 DGC has worked with all of these topics. This study concerns the first item. Electric heaters as supplement to existing gas boilers have been studied in . In this study an electric 2 kw heater was directly connected to the boiler return line. No storage facility was used. Finally, DGC has a long experience in cogeneration technologies. For example, for third-party testing, certification and field test evaluation in the Danish micro cogeneration program DGC is aiming at developing and demonstrating Danish gas-fired fuel-cell micro cogeneration units for single-family houses 3. The potential interruptible power demand when using hybrid systems can be illustrated as follows. Assume that hybrid systems consisting of an electric heat pump and a condensing boiler become a major replacement in the current Danish gas heating market. In this example we assume 100,000 installations, corresponding to one third of the gas-heated single-family house population. The heat pump in the hybrid system has a compressor input of 1 kw. The COP at the temperatures 7/35 4 is The power demand that can be replaced by gas heating as a function of the outdoor temperature then becomes as shown in Figure 2. The two dotted curves represent the total space heating demand in 100,000 houses with 10,000 and 20,000 kwh annual heating demand. The solid curves show the electricity demand for the heat pumps in the hybrid systems. The temperature where the horizontal electricity curves starts to decrease marks the lowest temperature where the heat pump alone can heat the house. At a lower outdoor temperature the heat pump will operate continuously on full load and the gas boiler will add the extra heat needed. The heat pump operates at part load at higher outdoor temperatures, and the interruptible power demand becomes lower. Please note that the heat pump is assumed to operate even though the outdoor temperature is very low. Some heat pumps are shut down when the outdoor temperature is for example -7 C or lower. New heat pumps can operate without this limitation during the entire year. 3 Dansk Mikrokraftvarme, 4 The temperatures 7/35 are the source temperature (air, soil etc) and the heating system forward temperature.
10 Heating and elec. demand (MW) DGC-report Overall heating demand, 20 MWH/a Interruptible elec. demand, 20 MWh/a Overall heating demand, 10 MWh/a Interruptible elec. demand, 10 MWh/a Ambient temperature ( C) Figure 2 Heating demand and interruptible power potential in Denmark when a population of 100,000 externally controlled hybrid systems are used 1.1 Hybrid system definition The definition of a hybrid heating system including a gas appliance is as follows: It consists of two appliances, one gas fired and one using another energy source, often electricity. The appliances can be separate appliances or integrated in the same package. The two appliances can independently cover the heating demand to a large extent. The appliances operation can be integrated or totally separated. An example of the first option is a heat pump, which can use the energy in the flue gases from the gas boiler. The operation of the two appliances shall be controlled either by internal or external signals. Some manufacturers have other definitions of a hybrid system, For example gas boiler and solar heating.
11 DGC-report Early work and appliances This section presents a few examples of early products and studies of the hybrid principle. The Swedish company IVT Elektro Standard manufactured a hybrid system already in the 1990s. The unit Auto Term 660A consisted of a noncondensing boiler and an electric heat pump. It was designed to integrate the heating and ventilation system. The heat source was the ventilation exhaust air and the flue gases when the boiler was in operation. The heat pump had an output of 2.0 kw and a COP of 2.9. The gas burner had three burner steps: 3.4, 7.2 and 10.6 kw. Heat pump operation was limited to return temperatures below 50 C. Measurements from installed units showed that the energy consumption was split into 40 % gas and 60 % electricity. An evaluation of the hybrid system is reported from the Swedish Gas Centre . At the International Gas Union Research Conference (IGRC) in 2008 Dutch laboratory tests of a hybrid system and micro cogeneration and a heat pump were reported . The principle of hybrid systems including heating and cooling were tested for early evaluation. Hybrid systems and smart grids are discussed in a paper from IGRC2011 . This paper discusses the effect on the electricity grid when a larger number of hybrid systems are in use. It thus deals with the aggregated grid aspects and not the individual appliance performance.
12 DGC-report 9 2 Current hybrid system appliances A number of hybrid systems are offered by manufacturers in Europe. In this chapter these appliances are described based on information from websites, journals and also a questionnaire sent out to manufacturers on the Danish market. 2.1 Products on the market The hybrid systems often consist of a gas boiler and an air-to-water heat pump in separate parts. There are also more integrated solutions and also prepared for other heat sources. The manufacturers often show system layouts, which also include solar energy. Figure 3 shows the main operation of a hybrid system. During times with high outdoor temperature the heat pump capacity is large enough to cover the heating demand. During winter the heating demand is covered by the gas boiler alone. The heat pump and the gas boiler are used simultaneously in an intermediate period. The heating costs using the condensing boiler and the heat pump are marked with red and green curves in the figure. The costs using the condensing boiler can be assumed not to differ significantly between various boiler models. The operation of the heat pump depends on the efficiency/cop and the lowest ambient temperature allowed for operation. The latter can differ heavily, from -5 C to for example -15 C for heat pumps adapted to the low winter temperatures in Scandinavia. The figure shows that in this configuration the heat pump will not be economical to operate when it is colder than -4 C. Several control strategies are implemented in hybrid systems. Some hybrid systems allow the customer to choose between two or more control options. These options include: Heat pump operation until the heating demand exceeds the heat pump s capacity. Ambient temperature may also restrict the heat pump operation. Operation to minimize the heating cost Operation to minimize the CO 2 emissions The two last options require preset values for gas and electricity prices, alternatively the price relation, and the CO 2 emission factors for gas and electricity. The control system calculates the cost or CO 2 emission and chooses
13 DGC-report 10 the appliance to be used in order to minimize the consumer energy cost or the overall CO 2 emission in an optimizing algorithm. These factors are input given by the installer or consumer and are fixed until new factors are given as input. Figure 3 Hybrid system operation (Source: Bosch) Boiler efficiency in a hybrid system will be lower than in a stand-alone installation due to the lower annual load and a higher return temperature if the heat pump heats the return line. The systems described in this chapter have mostly been found in internet searches for gas heating and hybrid. Manufacturers who do not use the expression hybrid for their hybrid systems may have been omitted. The following hybrid systems have an outdoor heat pump part and an indoor part and a boiler, unless otherwise stated Buderus The Buderus hybrid system is named Logatherm WPLSH. The indoor unit has the size mm (height width depth) and weighs 21 kg. The maximum supply temperature is 50 C. It operates in different modes. Hot water is always produced using the gas boiler. At low heating loads and moderate supply temperatures the heat pump alone operates. At low ambient temperatures the gas boiler is the only heat source. Between these operation modes is a range where the heating is split between the appliances, which
14 DGC-report 11 are operating at the same time. An existing boiler installation can be upgraded to a hybrid system if the boilers are the Logamax plus GB145, GB152, GB162, or GB172. Some Buderus boilers are sold in Denmark as Milton (Nefit) Highline. Buderus has used the word hybrid for combinations of gas boilers and solar collectors as well. Figure 4 Buderus Logatherm WPLSH hybrid system installation example Daalderop The Daalderop Cool is a single-unit wall-hung hybrid appliance. It uses both ambient air and ventilation exit air as heat sources. It can also be used for cooling purposes. The heat pump and the condensing boiler output is 3 and 24 kw, respectively. The condensing boiler assists the heat pump for heating during coldest days. The manufacturer states it is compact and easy to install with no buffer or separate storage tanks required. The size is mm (height width depth) and it weighs 110 kg. Figure 5 shows images of the appliance.
15 DGC-report 12 Figure 5 Daalderop Cube hybrid heating appliance design and installation example Vaillant Vaillant presented a hybrid system, geotherm, in March The heat pump has an output of 3 kw and can use ambient air and ventilation exit air as heat source. The heat pump is a single-unit indoor appliance. According to Vaillant the system is suitable both for existing and new houses. Gas boilers from Vaillant can be upgraded to a hybrid system by adding the heat pump. The gas boiler is used for hot water production. The image in Figure 6 shows from left to right the indoor heat pump, the hot water tank and the gas boiler. Figure 6 Vaillant geotherm hybrid heating system
16 DGC-report Junkers/Bosch Junkers presented two hybrid systems, CerapurAero and Supraeco SAS Hybrid, in April Junkers products are marketed under the Bosch name in Denmark. The hybrid systems have different layouts. They are shown in Figure 7. The CerapurAero hybrid system has the heat pump and condensing boiler in a common single cabinet. The size of this cabinet is mm (height width depth). The heat pump can use either ambient air or water as heat source. Heat pump output is approximately 2 kw and COP = 3.5 (7/35). The gas boiler has either 14 or 24 kw nominal output. The manufacturer claims an overall efficiency which is 12 % higher than for the gas boiler alone. The Supraeco SAS hybrid system has the indoor heat pump unit and the gas boiler in separate cabinets. The maximum output is 5.2 kw. Ambient air is used as heat source. The manufacturer states that it is possible to use the installed boiler in the Cerapur product series from 2007 for an upgrade of an existing heating installation to a hybrid system. Three operating strategies are possible: CO 2 reducing operation, Cost reducing operation Gas boiler operation below a preset ambient temperature The indoor unit size of the Supraeca is mm (height width depth).
17 DGC-report 14 CerapurAero indoor unit including heat pump and gas boiler Supraeco SAS indoor heat pump unit Supraeco SAS outdoor heat pump unit Figure 7 Junkers/Bosch hybrid heating systems CerapurAero and Supraeco SAS Viessmann Viessmann has no dedicated heating system or product called a hybrid system. However, the website mentions that a gas boiler and a heat pump can be connected in a common bivalent system. Figure 8 shows a picture from the website describing a system layout where an air-to-water heat pump is connected.
18 DGC-report 15 Figure 8 A hybrid system design consisting of a boiler, a heat pump and a storage tank suggested by Viessmann Glow-worm Glow-worm is a British hybrid system consisting of a condensing gas boiler and an air-to-water heat pump. The appliance is also sold as AWB Genia on the Dutch market. Figure 9 shows the system parts including control boxes and a remote control.
19 DGC-report 16 Figure 9 Glow-worm hybrid system parts The control system can set the operation in at least two modes, either as a capacity control or a price relationship control mode MHG MHG Thermipro is a floor-standing unit prepared also for solar energy use. The unit consists of a heat pump indoor unit, a condensing gas boiler and a 500 l storage tank and heat exchanger for solar energy. The heat pump output capacity is 7.1 kw with a COP = 4.2 (A2/W35) and the gas boiler is modulating in the range kw. The unit has a capacity significantly higher than most of the products presented in this chapter. Figure 10 shows a configuration. The left image shows an outside view of the hybrid unit. The middle image shows the interior parts, condensing gas boiler, heat pump indoor unit and storage tank for solar energy. The graph indicates that
20 DGC-report 17 the heat pump is not operating when the ambient temperature is below the freezing point. The heat pump and solar energy is used above the freezing point, while the gas boiler and solar energy are used below the freezing point. The right-hand graph shows an example of the annual distribution of energy as a function of the ambient temperature. The heat pump and solar energy covers 81 % of the annual heat demand and the gas boiler and solar energy cover 29 % of the annual heat demand. The manufacturer states that the gas consumption is low enough for LPG operation outside the gas grid area. The size is mm (height width depth) and the weight is 512 kg excluding water. Figure 10 MHG Thermipro hybrid system Carrier In the North American market the expression dual-fuel is often used instead of hybrid system. The Carrier Infinity dual-fuel hybrid system is an air-toair heat pump and a gas furnace (warm air) delivered either as an integrated package for outdoor location or as separate parts for indoor and outdoor location. The system provides heating and cooling including humidifying the warm air for space heating. Figure 11 shows the two options with the separate design to the left and the integrated package in the right image.
21 DGC-report 18 Figure 11 Carrier Infinity dual-fuel hybrid systems 2.2 Summary of hybrid systems In Table 1 the main characteristics of hybrid systems are collected. Two products not described above are included in the table to show that the description is not complete. Table 1 Summary of European packaged hybrid systems Manufacturer/Model Heat source Heat pump output, COP Possible to add heat pump to existing boiler? Buderus Logatherm WPLSH Ambient air kw COP = 4.42 A7/W35 Can be combined with several Buderus gas boilers Daalderop Cube/Cool Cube Ambient air 3 kw No Vent exit air COP = 2.9 Brötje/Bosch/Junkers Cerapur Aero Supraeco SAS Ambient air/ water 2 kw, COP = kw, COP = 4.11 Yes (Supraeco) Viessmann Ambient air - Yes Vaillant Ambient air 3 kw Yes Vent exit air Glow-worm Clearly Hybrid Ambient air 4.41 kw - (AWB GeniaHybrid) COP = 3.73 A7/W35 MHG ThermiPro Ambient air (solar energy) 7.2 kw COP = 4.2 A2/W35 No, delivered as a complete package Carrier Infinity Dual Fuel System Ambient air US energy efficiency labels. SEER = 15.0 (heat pump) AFUE >81 % (gas Delivered either as package or heat pump and gas furnace separately
22 DGC-report 19 boiler, HCV) HSPF = 8.0 Techneco Elga Ambient air 5 kw Can be combined with boilers from a number of manufacturers Liechti EcoStar Hybrid ThermiAir Hybrid Ambient air kw Oil boiler Gas and electricity meters are an essential part of the smart-grid concept. To use the full potential of external monitoring and control in order to fit into the smart-grid concept the gas and electricity meters need to be equipped with two-way communication. These options are not necessary for the operation of a hybrid heating system. In the future, smart meters with communication between the heating system and external connections will facilitate operation based on dynamic energy prices and correct billing. Today, only preset fixed values for gas and electricity can be used for price control of the hybrid system operation. The gas and electricity utilities will also get an overview of the electricity demand in real time and the possible interruptible electricity power.
23 DGC-report 20 3 Hybrid model and calculations The annual performance of a hybrid system is modelled and simulated in this chapter. The system consists of an electric air-to-water heat pump and a gas boiler. The calculations can only include internal control systems. The two control systems used are: The heat pump is the primary heat source. The gas boiler is used when the heat pump output is not large enough for the heating demand. The appliance chosen for heating is based on the price difference between natural gas and electricity. The calculations are made using the model (Boilsim) for energy labelling of gas boilers in Denmark. A simplified heat pump model is incorporated in the Boilsim calculations. The Boilsim model is described in . The Coefficient of Performance (COP) for the heat pump is modelled as COP AT out BT sup ply where T source is the source temperature at the evaporator, and T supply is the supply temperature to the heating system. The compressor input is also a calculation input and used together with the COP to calculate the heat pump output in each climate step. The coefficients A and B are fixed and describe the heat pump performance map. An example of a heat pump performance map is shown in Figure 12. The source temperature is equal to the outdoor air temperature, and is input in the climate steps. T supply is calculated in each climate step. The shape of the performance map in Figure 12, and the fact that heat pumps are often characterized by the COP in only 2-3 operating points, show that modelling the COP as a plane is a reasonable simplification.
24 DGC-report 21 Figure 12 Performance map for an air-to-water heat pump  3.1 Calculation parameters Input to the calculation of annual performance and heating costs for hybrid systems are summarized in Table 2. Table 2 Technical parameters Parameters in hybrid system performance calculations Heat pump COP (A7/W35) 3.85, 4.25 and 4.65 Heat pump compressor input Heat pump output (7/35) Operation control Gas boilers Economic parameter Price relation electricity/gas 1.0 and 1.5 kw 3.85 kw, 4.25 kw and 4.65 kw (1.0 kw) 4.81 kw, 5.31 kw and 5.81 kw (1.5 kw) 1. The gas boiler is used for hot water production and peak load 2. The gas boiler is used for hot water production and when the heating cost is lower for the boiler. Two condensing gas boilers with modulating burners, 3 15 kw (boiler 1) and 6 25 kw (boiler 2). Annual efficiency in the Danish energy labelling system: % at 20 MWh annual heating demand and 2,000 kwh hot water demand. 2.7 (only used for capacity controlled operation) in base case. 2.2 and 3.2 in calculations for price relation sensitivity
25 DGC-report 22 The calculation procedure is as follows. The calculations are made in the same manner as the calculations of annual efficiency for the Danish energy labelling system. The Boilsim model is used. The heating season is divided into 13 climate steps, representative of the Danish climate. For each of these climate steps the operation conditions are calculated, i.e. the heat demand and the supply and return temperatures. Calculations are made for an annual heating demand of 10, 20 and 30 MWh. The annual hot water demand is 2,000 kwh. The gas boiler is supposed to cover the entire hot water demand. The heat pump is assumed to be able to operate down to -15 C ambient air temperature. Two operational cases are calculated as: Capacity controlled operation. Heat pump COP is calculated using the ambient air temperature and the heating system supply temperature. The calculated COP and heat pump output is compared to the heat demand. If the heat pump capacity is not enough, the gas boiler adds the remaining heating part. Gas boiler efficiency is calculated for this reduced heat demand. Energy price controlled operation. The heat pump COP and the gas boiler efficiency are calculated for the heating demand. If the relation between the calculated heat pump COP and gas boiler efficiency is less than the price relation between electricity and gas, the gas boiler covers the entire heat demand. The heat pump covers the heat demand if the result is the opposite. If the heat pump output is not sufficient, the gas boiler operates as in the capacity controlled operation. The price relation between electricity and gas is given as a fixed input. The effect of dynamic prices set by the electricity utility cannot be calculated with the method. The 12 heating systems in the base case calculations are described in Table 3.
26 DGC-report 23 Table 3 System layout for hybrid system evaluation System Boiler Heat pump electricity input (kw) (A7/35) Sys 1 1 (3-15 kw) Sys Sys Sys Sys Sys Sys 7 2 (6 25 kw) Sys Sys ,65 Sys Sys Sys The air-to-water heat pump performance map is shown in Figure 13. Actual COP in the calculations are shown in the graphs showing the annual system performance. Figure 13 Performance map of air-to-water heat pump used in calculations of hybrid system annual performance The results are presented as graphs with the annual gas consumption as function of price relation, heat pump performance (COP) and annual heating demand. The energy demand on the x-axis is the space heating demand. In the calculations an annual hot water demand of 2,000 kwh is added. The
27 DGC-report 24 heating system efficiency is expressed as a weighted efficiency with an upper and lower limit. The lower limit is calculated with electricity energy multiplied by a factor 2.5 to get the primary energy consumption. The electricity consumption is treated in the same manner in the current Danish energy labelling system. The upper limit is calculated as if the electricity generation is carbon-free, for example wind power, solar power or hydro power. The box at the upper right corner contains information about the heat pump performance and the electricity input to the compressor. The number at each point of the weighted annual efficiency (green dotted line) is the heat pump seasonal COP calculated as the heat pump output divided by the electricity consumption. An electricity consumption of 100 W is assumed for fans, pumps and heat pump electronics. The heat pump COP is assumed to be constant regardless of the output, i.e. the part-load COP is equal to the fullload COP. It should be noted that the electricity consumption is the sum of electricity to the heat pump compressor and electronics, fans and circulation pumps. The latter part is approximately 500 kwh. Thus the electricity consumption in the graphs cannot be multiplied by the adjacent COP to get the output energy from the heat pump. The gas consumption is composed of gas consumption for hot water production and heating when the heat pump is not covering the entire heating demand. The hot water demand is 2,000 kwh annually in all calculations. For the boilers chosen the hot water efficiency is approximately %. This means that 2,700 kwh gas is consumed for hot water production and is not affected by the heat pump operation. The operating conditions for the gas boiler are different from the situation when a boiler alone covers the heating demand. Firstly, the return temperature increases as the pump operates and, secondly, the boiler load is reduced. As a result the boiler efficiency decreases. For the 20 MWh case the annual efficiency is decreasing to approximately 95 %, which leads to an increase in gas consumption. This is equivalent to kwh annual increase in gas consumption. 3.2 Gas boiler operates as peak load capacity control The graphs in Figure 14 and Figure 15 show from top to bottom systems with the same gas boiler but with heat pumps with increasing COP. The
28 DGC-report 25 result is an increased heat pump output and decreased gas consumption. The graphs in Figure 15 where the compressor input is 50 % higher (1.5 kw) show that this tendency is further enhanced. Figure 14 and Figure 15 show the result for boiler 1 (3-15 kw) and air-towater heat pumps with 1.0 and 1.5 kw compressor input, respectively. The three graphs in each figure show an increasingly higher heat pump COP and follow a vertical line between the performance maps in Figure 13. The graphs show performances for hybrid systems including boiler 1 with a modulating range of 3-15 kw. Boiler 2 with 6-25 kw modulating range shows no significant differences compared to the results shown in Figure 14 and Figure Price controlled operation The results for calculations where the hybrid system operation is controlled by the price relation between electricity and gas are shown in Figure 16 and Figure 17. The current price relation for gas and electricity has been used. In this control strategy the control system will actively move heat production from the heat pump to the gas boiler in order to minimize the customer s heating bill. In a real installation where this option is possible the boiler efficiency and the heat pump COP are in some way compared. No applicable algorithms have been found. If the capacity control results are compared to the corresponding price controlled results we observe slightly lower electricity consumption when a price controlled operation is used. This is more visible for the lower annual heating demands where the heat loads are lower.
29 DGC-report 26 Figure 14 Annual performance for hybrid system - Boiler 1 and 1 kw compressor input. Capacity controlled operation.
30 DGC-report 27 Figure 15 Annual performance for hybrid system - Boiler 1 and 1.5 kw compressor input. Capacity controlled operation.
31 Gas and electricity consumption (kwh) Annual efficiency (%) Gas and electricity consumption (kwh) Annual efficiency (%) Gas and electricity consumption (kwh) Annual efficiency (%) DGC-report Price control - Energy consumption and annual efficiency. El/gas price = 2,7 Gas consumption Electricity consumption Weighted annual efficiency Annual efficiency, green elec COP A7/W35 3,85 A0/W55 2,55 P comp ,40 3,44 3, Annual space heating demand (MWh) + 2 MWh hot water Price control - Energy consumption and annual efficiency. El/gas price = 2,7 Gas consumption Electricity consumption Weighted annual efficiency Annual efficiency, green elec COP A7/W35 4,25 A0/W55 2,95 P comp ,55 3,63 3, Annual space heating demand (MWh) + 2 MWh hot water Price control - Energy consumption and annual efficiency. El/gas price = 2,7 Gas consumption Electricity consumption Weighted annual efficiency Annual efficiency, green elec COP A7/W35 4,65 A0/W55 3,35 P comp ,84 3,94 4, Annual space heating demand (MWh) + 2 MWh hot water Figure 16 Annual performance for hybrid systems - Boiler 1 and 1 kw compressor input. Price controlled operation.
32 Gas and electricity consumption (kwh) Annual efficiency (%) Gas and electricity consumption (kwh) Annual efficiency (%) Gas and electricity consumption (kwh) Annual efficiency (%) DGC-report Price control - Energy consumption and annual efficiency. El/gas price = 2,7 Gas consumption Electricity consumption Weighted annual efficiency Annual efficiency, green elec COP A7/W35 3,85 A0/W55 2,55 P comp ,27 3,40 3, Annual space heating demand (MWh) + 2 MWh hot water Price control - Energy consumption and annual efficiency. El/gas price = 2,7 Gas consumption Electricity consumption Weighted annual efficiency Annual efficiency, green elec COP A7/W35 4,25 A0/W55 2,95 P comp ,55 3,57 3, Annual space heating demand (MWh) + 2 MWh hot water Price control - Energy consumption and annual efficiency. El/gas price = 2,7 Gas consumption Electricity consumption Weighted annual efficiency Annual efficiency, green elec COP A7/W35 4,65 A0/W55 3,35 P comp ,86 3, , Annual space heating demand (MWh) + 2 MWh hot water Figure 17 Annual performance for hybrid systems - Boiler 1 and 1.5 kw compressor input. Price controlled operation.
33 DGC-report Energy costs for the hybrid system The annual heating costs for hybrid systems are illustrated in two graphs in Figure 18. The upper graph shows the costs for capacity controlled systems and the lower graph shows the results for price controlled systems. The solid blue graph at the far right shows the heating cost for a gas boiler alone. Danish energy prices have been used. The gas price is 8.12 DKK/m 3 and the electricity price used is 2.04 DKK/kWh. The graphs clearly show that the heating cost can both lower and higher than the heating cost for a condensing boiler alone. Price controlled operation shows slightly lower heating cost where the gas cost has a larger share of the overall cost than in capacity controlled operation mode. Figure 18 Heating cost for hybrid systems installed in a house with 20,000 kwh annual heat demand and 2,000 kwh hot water demand
34 DGC-report 31 The influence of the price relation (e/g) between electricity and natural gas is illustrated in Figure 19. Heating with a gas boiler only has the heating cost equal to 1.0. Systems 4 6 are selected for this calculation, i.e. boiler 1, 1.5 kw compressor input and heat pump COP = 3.85, 4.25 and Figure 19 Influence of the price relation (e/g) between electricity and natural gas on the relative heating cost using hybrid systems compared to a condensing gas boiler alone The calculations show that the lowest heating cost for the consumer is obtained when the electricity price is favourable and thus maximizing the operation with the most efficient heat generator, the heat pump. A possible reduction of up to 30 % of the heating cost is possible with the input data used. However, current Danish gas and electricity prices rather indicate a possible reduction of % for the consumer. Only the cost for gas and electricity is reasonably known today. Due to the early market stage the investment and maintenance costs are not known well enough to make an overall economic evaluation for the consumer. This ought to be assessed in further studies and field tests. The advantages for the gas and electricity utilities lie mainly in the possibility of using the hybrid systems as part of a smart grid where gas and electricity interact. Renewable electricity can be used in the best possible way. The hybrid systems can be switched over to gas operation when for example the wind power output is low and the option is expensive power generation with high CO 2 emissions.
35 DGC-report 32 4 Conclusions Hybrid systems have recently been introduced on the market. An electric heat pump covers the base load while a gas boiler covers peak heating load and the hot water production. These heating systems are offered either as integrated units, separate heat pump and boiler or as add-on heat pump to existing gas boilers. The hybrid system operation may be controlled according to several principles. Most common is a capacity controlled operation where the gas boiler is used for heating only when the heat pump output is not large enough for the heating demand. In a price controlled operation the lowest heating cost determines heat pump or boiler operation. The heat pump COP is checked against the boiler efficiency and the relationship between electricity and gas price. In a similar way the lowest CO 2 emission can control the operation. In the future, an external control by gas or electricity utilities as part of a smart-grid concept will be possible in order to adapt the operation to the availability of renewable electricity. In this report hybrid systems are described and the operation is simulated. These simulations show the annual gas and electricity consumption for capacity and price controlled operation as a function of heat pump COP, size and the price relationship between electricity and gas. The heating cost is not automatically reduced when hybrid heating systems are used compared to a condensing boiler only. The heat pump COP and energy prices play an important part in the economic bottom line for the consumer. If the heat pump COP is not high enough and the electricity price is not favourable the heating cost may even increase. These calculations show that the lowest heating cost is obtained when a price control mode is used and the electricity price is favourable, thus maximizing the operation with the most efficient heat generator, the heat pump. The calculations in this study show a possible reduction of up to 30 % of the heating cost. However, current Danish gas and electricity prices rather indicate a possible reduction of % for the consumer. This ought to be verified by controlled field tests. The investment and maintenance cost for the
36 DGC-report 33 consumer are today not known due to the fact that hybrid systems are only at a market introduction stage. The advantages for the gas and electricity utilities lie mainly in the possibility of using the hybrid systems as part of a smart grid where gas and electricity interact. Renewable electricity can be used in the best possible way. The hybrid systems can be switched over to gas operation when for example the wind power output is low and the option is expensive power generation with high CO 2 emissions.
37 DGC-report 34 References  EU Commission, "Smart Grid Aspects related to Gas," Task Force for Smart Grids. Expert Group 4,  M. Larsen, "Regulerkraft på villakedelområdet. Overskuds-el fra vindmøller til naturgaskedler," Dansk Gasteknisk Center,  J. Hult and J. Jonasson, "Jämförelse mellan konventionell gaspanna och värmepump med inbyggd gaspanna (In Swedish)," Swedish Gas Centre, SGC 113,  J. Lemmens, J. Darmeveil, J. Eerland, S. Hegge, J. Turkstra and J. Westing, "Combination micro-chp - heat pump: proof of principle," in International Gas Union Research Conference IGRC2008, Paris,  C. Vuillecard, "Bottom-up model for local gas and electricity interactions with hybrid technologies," in International Gas Union Research Conference, IGRC2011, Seoul,  T. Afjei and R. Dott, "Heat pump modelling for annual performance, design and new technologies," in 12th Conference of International Building Performance Simulation Association, Sydney, November 14-16,  H. Cheung and J. E. Braun, "Performance Characteristics and Mapping for a Variable-Speed Ductless Heat pump," in International Refrigeration and Air Conditioning Conference, Purdue University,  L. van Gruijthuijsen and M. Näslund, "Description of the calculation method for the Danish labelling of gas boilers. ver 2," Danish Gas Technology Centre, DGC, 2010.
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