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1 Chalmers tekniska högskola VÄRMEPUMPAR I VATTENBURNA VÄRMESYSTEM – Effektiva lösningar med värme och varmvatten vid konvertering av elvärmda småhus eff-Sys H23Mr Chairman, ladies and gentlemen, my presentation this morning deals with possibilities of IMPROVING THE EFFICIENCY OF HYDRONIC HEAT PUMP HEATING SYSTEMS.My focus will be on retrofit applications in houses with direct acting electric heating. In this case GSHPs are installed with a very small hydronic heating system commonly known as a miniwater system and this has some specific challenges.The basic ideas, however, are generally valid but some of the problems will be less pronounced in other applications.Now, first a brief background….Per FahlénChalmers tekniska högskola
2 DELTAGARE Chalmers tekniska högskola, Installationsteknik Boröpannan ABElforsk ABGrundfos ABIVT ABNibe ABSP Sveriges Provnings- och Forskningsinstitut ABThermia Värme ABThermopanel ABWilo ABIn Sweden, boilers are being replaced by heat pumps at a substantial rate in hydronic heating systems. As a result, for many years a heat pump has been the most frequently chosen individual heating system.There is, however, more political interest in replacing electric heating. This will also be more interesting in the future from a market point of view and there will also be more interest in full coverage heat pumps. Hence the interest in replacing direct-acting electric heating.This photo shows a typical Swedish house with electric heating. It is timber framed, prefabricated, a low heating demand and, of course, no traditional heat distribution system. Therefore, it needs a new miniwater system.Looking at the sales figures, note the change in relation between air and GSHP (decrease in building/EA hp, decrease in A/A). You can also see that the Nordic heat pump competition in 1995 had a large impact.Sales reflect boiler replacement. However, the retrofitting of electric heating is an altogether different proposition. So, what are the obstacles to overcome?
3 BAKGRUND Konvertering av pannor Konvertering av direktel Vattenburna systemKonvertering av direktelInget distributionssystemNytt ”minivatten” systemVärmepumps-försäljningVärmepumpstävlingenMarkvärme dominerarIn Sweden, boilers are being replaced by heat pumps at a substantial rate in hydronic heating systems. As a result, for many years a heat pump has been the most frequently chosen individual heating system.There is, however, more political interest in replacing electric heating. This will also be more interesting in the future from a market point of view and there will also be more interest in full coverage heat pumps. Hence the interest in replacing direct-acting electric heating.This photo shows a typical Swedish house with electric heating. It is timber framed, prefabricated, a low heating demand and, of course, no traditional heat distribution system. Therefore, it needs a new miniwater system.Looking at the sales figures, note the change in relation between air and GSHP (decrease in building/EA hp, decrease in A/A). You can also see that the Nordic heat pump competition in 1995 had a large impact.Sales reflect boiler replacement. However, the retrofitting of electric heating is an altogether different proposition. So, what are the obstacles to overcome?
4 PROBLEM ”Minivattensystem” med fläktkonvektor Låg termisk massaStartfrekvens, drifttidRumstemperatur-regleringVattentemperaturunder driftParasiteffekterPumpar (värme- och köldbärare, återladdning)Fläkt (fläktkonvektor)Economy has resulted in the use of minimized installations with small hp, short borehole and a miniwater heating system with a fan-coil.Such a system has a low thermal mass which results in problems with high starting frequencies, under par room temperature control and high on-time heating water temperatures. These problems are significant with a mini water system but they also become more noticeable in traditional systems as the relative heat pump size increases as with full energy coverage.Another issue is the influence of parasitic drive powers. A number of pumps and fans degrade COP through poor efficiency and long operating hours.This presentation will illustrate these problems through a case study. In connection with the aforementioned hp competition, 5 houses were retrofitted with the winning concept and evaluated over 5 years. One house was then equipped with a recharging system for the borehole to see the effect on brine temperature.
5 VÄRMESYSTEM Original: Direktverkande elvärmeradiatorer Konvertering 1: Bibehållen direktelvärmeMinivattensystemmed kurvstyrningKonvertering 2:Bufferttank med tapp-varmvattenUtetemperatur ochrumsstyrningThe original heating system comprised 15 direct-acting electric heaters with individual room-thermostats and a total capacity of 13.2 kW.The 1st retrofit included a GSHP from heat pump competition with a miniwater system based on 1 fan-coil with curve control, i.e. an outdoor temperature related return water temperature. The pump and fan run continuously. Focus on low cost and the electric heaters were retained as back-up. The system works but there are many opportunities for improvement.Three major system factors degrade performance: raised part-load heating system temperature, parasitic drive powers and inefficient controlThe first can be addressed with load matching (capacity control, storage), the second by selection of more efficient components and the third by demand control.The 2nd modification includes load matching by means of a TES, SHW (economy), new control including direct temp related speed control of pumps and more efficient pumps.I should add that a hydronic rather than an air system is chosen because of SHW.
6 VÄRMEPUMPSSYSTEM Återladdning: Komfortkyla Värmeåtervinning - frånluftsbatteri TILL- tilluftsbatteri FRÅNKomfortkyla- frånluftsbatteri FRÅN- tilluftsbatteri TILLVärmeåtervinningNow, turning to the heat pump system, the 1st retrofit had quite a short boreholeThe 2nd retrofit includes recharging of the borehole with exhaust air. This was installed in 2000 and was planned from the start in 1995 to see the effect on borehole temperature. For many reasons, recharging by exhaust air is more viable than by solar systems.This upgrade also provides the possibility of free cooling and highly efficient heat recovery. The diagram illustrates the principle modes of operation. (It is included in this presentation because it emphasizes the problem of capacity over thermal mass. It also illustrates that one must address the entire system when proposing improvements. Otherwise an improvement may end up being a degradation.)So, what are the practical results?
7 RESULTAT: DRIVENERGI OCH COP Beror av antal enheter, drifttid, verkningsgrad, temperatur och styrningCOP: Exempel med återladdning (tute = 5 °C)tkbin ökar 3 K,COP minskar 11 %!COP = 4.2→3.5→ 3.0 →2.6 (tvbut , We,p , We,f)Drive power depends among other things on component efficiency, temperature of source and sink and the control strategy and any increase will directly degrade the COP.Degradation acts by several mechanisms: e.g. directly from parasitic drive powers and indirectly through increased capacity. This results in a higher condensing temperature and a shorter on-time which further raises the temperature.The efficiency of pumps and fans are much below state of the art and this may be cheaper to improve than compressor efficiency. Also, their continuous operation results in unnecessary drive power, or parasitic power as it were.The results highlight the importance of matching capacity with demand (temp level)The results also highlight the benefit of reducing parasitic drive powers. You can see this in the diagram as you move from COPhp-COPhps-COPhpsf.As an example, let’s take a look at the effect of recharging. At an outdoor temperature of e.g. +5 °C, recharging raises tb,i by 3 K but reduces COPhp by 11 %! This reduction is contrary to the purpose of recharging but expected since the raised brine temperature increases the heat pump capacity and hence raises the on-time heating temperature by two mechanisms (see 3 Theory and 4 Discussion).At the outdoor temperature +5 °C, the brine inlet temperature is close to +5 °C. Based on laboratory tests, at this condition the heat pump should deliver a steady state COPhp of 4.2 and a COPhps, including pumps, of 3.6. Hence, even under ideal conditions the internal parasitic drive powers reduce COP by 14 %. Using pumps with a state-of-the-art efficiency of 40 %, compared to the present level of less than 10 %, would raise COPhps from 3.6 to 4.05.Now, let’s turn to the temperature levels.
8 RESULTAT: TEMPERATURER VärmevattenI medelUnder driftKöldbärareÖkat med 3 KFrånluftAs can be seen in both diagrams, most of the heat is delivered at a mean temperature below 35 °C. The on-time temperature, however, is higher most of the year. The lower diagram indicates the relation between heat delivery and temperature.Recharging generally raises the brine temperature by approx. 3 K and it remains above 0 °C all the time.The diagram also indicates the efficiency of the exhaust-air heat recovery coil. In winter, the exhaust-air is cooled from +20 °C down to almost 0 °C continuously with no frosting of the heat exchanger. Since the outlet brine temperature of the heat pump is fed directly to the coil, the exhaust-air can be cooled to a level below the inlet brine temperature to the heat pump.Now we come to the main points of this presentation, i.e. The fan-coil and heat pump operation.
9 RESULTAT: FLÄKTKONVEKTOR StyrningKonvektorfläkt tillVärmepump till-frånTemperaturTilltemperatur > medeltemperaturExempel: tute = 4,4 °CControl is rudimentary and the fan-coil operates continuously in the sense that water flow and airflow are constant. The temperature, however, has a cyclic, transient pattern due to the on-off operation of the heat pump. The diagram illustrates the variation over a full on-off cycle at an outdoor temperature of +4 °C After switching on or off the heat pump, there is a delay time, which corresponds to the time it takes to circulate the water once around the heating system.The diagram also indicates that the heating system is on, but at a lower rate, during the heat pump off-period. Thus switching off the water pump during the heat pump off-period would diminish the heating system on-time and hence further raise the heat pump on-time temperature level. The mean on-time heating water temperature is 5.7 K higher than the cycle mean value. This would typically imply a reduction of COPhp by % but for this particular heat pump the reduction exceeds 20 %.The second diagram compares the actual temperatures with the ideal temperature, which could be achieved by perfect load matching. In this case the difference is more than 6 K with a correspondingly even greater difference between actual and theoretical COPhp. The thermal efficiency of the fan-coil unit was calculated from laboratory tests and subsequently verified from in situ measurements to be e = 0.73.Obviously the transient heating system characteristics will influence hp operation.
10 RESULTAT: VÄRMEPUMPSDRIFT Relativ gångtidApprox. linjär med tuteför tute > tbalansStartfrekvensMax beror av värmepumpens relativa storlekDrifttid per startMonoton minskning med tuteThe relative operating time reflects the size of the heat pump in relation to the design heat load of the building. The relative operating time of the heat pump is a more or less linear function of outdoor temperature until the balance temperature is reached after which it equals 1. The starting frequency and the operating time per start, however, will depend also on the time constant of the heating system. The first diagram shows that the starting frequency has a peak, which will be higher and occur at a higher outdoor temperature the smaller the time constant of the heating system is. The operating time per start, on the other hand, has a monotonous increase with decreasing outdoor temperature as indicated in the second diagram.These practical results can be explained by simplified theoretical considerations.
11 TEORI: VÄRMEBEHOV Byggnad (klimat, komfort, klimatskärm, internlast) TillförselRelativgångtid:LagringÖkad gångtid:The heat demand depends on the loss factor of the building, Khouse, the desired indoor temperature, troom, the equivalent internal heat gains, qgain, and the outdoor temperature, tout,In this case, the result is an accumulated heat demand of 16 MWh/year. It should be noted that although the annual mean temperature is +6 °C and the mean temperature during the heating season is +2 °C, the maximum number of degree hours per temperature interval occurs at –4 °C as can be seen in the first diagram. In a Swedish climate, typically % of the heat demand is covered by a capacity which is less than 50 % of the design capacity. Hence the relative operating time of the heat pump, given by this eq. , will be less than 0.5 most of the time if the heat pump system is monovalent.When supply exceeds demand, the size of the heat pump in relation to the heat demand of the house will decide Rhp.Storage can be used to increase the operating time per start and the cycle time, and hence reduce the starting frequency. Ton will be determined by the thermal capacity of the heating system, Qhs, consisting of the room-heater capacity, Qrh, and a possible storage tank with Qhs = Qtes + Qrh.The storage capacity as well as the room-heater capacity will depend on the outdoor temperature controlled heating water supply temperature and as a consequence the starting frequency will have a maximum value.
12 TEORI: VÄRMESYSTEM, TRANSIENT DödtidTransport- och genomloppstidTidkonstantTermisk massa för konvektorVärmeöverföringskapacitetVärmekapacitetsflödenThe transient behaviour of the heating system depends on the relation between heat pump capacity, the room heater capacity and thermal mass, the distribution efficiency (coupling between room heater and building) and the building heat demand and thermal mass.There are two important dynamic characteristics, dead time and time constant.Ataer,Ileri, Gögys, have developed a simplified model which is quite useful. It highlights the factors which influence the transient behaviour.Sensible to use flush time as time unit since the ratio between dead time and time constant defines control difficulty.Below the balance temperature, however, performance will depend primarily on the steady-state characteristics of the heating system.
13 TEORI: VÄRMESYSTEM, STATIONÄRT VärmeöverföringskapacitetVärmeöverföringsenheterVärmekapacitetsflödenDistributionseffektivitetThe steady state temperature level depends on the relative capacity of the heat pump/room heater/building plus the distribution efficiency (coupling room heater/building). The room-heater capacity is given by the number of transfer units and the air and water heat capacity flowrates. The driving temperature difference will be the difference between the inlet water and air temperatures.Please note that Ta1 = Troom is only true if the distribution efficiency = 1, I.e. no short circuiting between the air outlet and inlet. A poor distribution may appear as a decreased time constant and reduced heat transfer capability of the room heater.The characteristics of the building and the heating system will decide the operating conditions of the heat pump.
14 TEORI: VÄRMEPUMPSDRIFT Inverkan på startfrekvensRelativ storlek (värmepump, värmesystem, byggnad)Inverkan på COPDrivenheter och temperaturnivåer:Exempel med återladdningDTkb = +4 K borde ge DCOP/COP ≈ +10 %Men DTvb >+4 K, Dwe,p/we,vp = ≈ -43 %Total minskning %!For a given heat pump, the basic temperatures will decide the capacity and COP.The starting frequency depends on the heat pump size in relation to the heating system and building and hence if you affect the temperature levels you will also affect the starting frequency and operating times.In addition to temperature, COP will directly depend on the efficiency of drive units such as compressor, pumps, and fans. Hence it will also be affected by changes in the number of drive units and their efficiencies.Assuming that the compressor efficiency is constant within modest changes in pressure ratio, the temperature dependence of the actual COP1 will be close to that given by the theoretical Carnot efficiency and changes in evaporating (T2) and condensing (T1) temperatures will follow changes in brine (Tb) and heating water (Tw) temperatures. Thus the eq. indicates how changes in the heating water and brine temperatures as well as changes in parasitic drive powers to brine pump, recharging pump, heating water pump and fan-coil, affect COP1As an example, recharging affects total COP1 positively by increasing the brine and evaporating temperatures Tb and T2 and hence reducing the compressor electric motor input for a given heat extraction. However, increased capacity raises the condensing temperature and additional pump and fan work to the coil also has a negative effect. In my previous example, for instance, recharging raised the brine temperature by 4 K. This could improve COP1 by 10 % at constant Tw,o. The ensuing increase in capacity, however, will raise Tw,o enough to almost totally annul the improvement unless the heating system is also upgraded. At the same conditions, continuous operation of the heating water pump will reduce COP1 by 9 %, the recharging pump by 10 % and the fan-coil fan by 24 %. Furthermore, the excessive on-time heating water temperature will reduce COP1 by 12 %, assuming the present low efficiencies of pumps and fans.So, to summarize my presentation:
15 DISKUSSION Teori och praktisk erfarenhet visar att: Stor värmepump kan orsaka problem med effektivitet och tillförlitlighet (tendens till högre täckningsgrad)Förbättringar på ett område måste kompletteras med andra förbättringar för att få effektDagens små pumpar ochfläktar har låga verknings-grader ( %)Potential för förbättradstyrning och reglering…………Some of these problems can be solved by load-matching.…………E.g. An improved heat source must be balanced by an improved heating system.Still, a conclusion is that …
16 SLUTSATS Markvärmesystem kan användas vid konvertering av direktelhus Lastanpassning förbättrar värmepumps-systemets effektivitet och tillförlitlighetViktigare vid ökande täckningsgradKapacitetsreglering eller ackumuleringIntegrera lagring (värmesystem och tappvatten)Reduktion av parasiteffekter ökar effektivitetenAnvänd bästa teknik för pumpar och fläktarSe över styrstrategin…… This is valid even with simple solutions and cost is important.…… It is particularly important in heating systems with low thermal mass and may improve efficiency by >10 %, and in the process enhance the reliability.… State of the art technology for pumps and fans can improve COP by > 15 %Load matching and reduction of parasitic drive powers may be more cost-effective than corresponding improvements of the heat pump per se.