1、PDF外文：http:/ 录 附录 A 外文资料 Retrofitting Domestic Hot Water Heaters for Solar Water Heating Systems in Single-Family Houses in a Cold Climate:A Theoretical Analysis Abstract: One of the biggest obstacles to economic profitability of solar water heating systems is the investment
2、 cost. Retrofitting existing domestic hot water heaters when a new solar hot water system is installed can reduce both the installation and material costs. In this study, retrofitting existing water heaters for solar water heating systems in Swedish single-family houses was theoretically investigate
3、d using the TRNSYS software. Simulation models using forced circulation flow with different system configurations and control strategies were simulated and analysed in the study. A comparison with a standard solar thermal system was also presented based on the annual solar fraction. The simulation r
4、esults indicate that the retrofitting configuration achieving the highest annual performance consists of a system where the existing tank is used as storage for the solar heat and a smaller tank with a heater is added in series to make sure that the required outlet temperature can be met. An externa
5、l heat exchanger is used between the collector circuit and the existing tank. For this retrofitted system an annual solar fraction of 50.5% was achieved. A conventional solar thermal system using a standard solar tank achieves a comparable performance for the same total storage volume, collector are
6、a andreference conditions. Keywords: solar thermal storage tank water heater retrofit domes 1. Introduction One of the biggest obstacles to the economic profitability of domestic solar water heating (SWH) systems is the investment cost . The installation cost of forced
7、 circulation systems used in cold climates can represent up to 50% of the total investment cost depending on the size and type of system . Also, the solar storage is one of the most expensive components in a solar water heating system . Retrofitting existing domestic water heaters when new SWH syste
8、ms are installed can reduce the total investment cost by decreasing both the installation and material costs. In Sweden there are more than half a million electrically heated single family houses that use conventional water heaters for domestic hot water production . Such retrofitting needs to be ca
9、rried out with consideration of the cold Swedish climate. In such regions the solar storage is placed indoors and a freeze protection medium runs inside the collector circuit. Since existing water heaters are not provided with a heat exchanger, an external one should be used. There are two main type
10、s of systems designed with an external heat exchanger outside the storage: thermosyphon and forced flow circulation . Cruickshank and Harrison, in 2004 and 2006 , investigated this type of thermosyphon systems in the Canadian cold climate. In 2011 , the same authors studied the performance of series
11、 and parallel connected thermosyphon storages. Thermosyphon systems became popular in several parts of the world such as Eastern Asia and Australia, mainly due to their simplicity and reliability . The thermosyphon driving force depends on the pressure difference and frictional losses between the he
12、at exchanger side-arm and the tank. Hence, the generated flow will be a complex function of the state of charge of the tank, the temperature profile along the heat exchanger and pipes, the height difference between the top of the heat exchanger and the top of the tank and the pressure drop in the he
13、at exchanger, piping and connections . Such dependence on the heat exchanger pressure drop and tank characteristics limits how the retrofit is carried out, where the heat exchanger should be placed and which storage tanks can be used . Moreover, when properly designed, forced circulation systems can
14、 significantly achieve higher performances compared with natural convection driven systems . This is mainly explained by the increase in energy transfer rate at a low energy driving cost. For example, a 40 W pump can generate a driving force 45 times higher than the one achieved by natural convectio
15、n systems. In addition, low energy pumps are now available at a lower cost. The disadvantages of forced circulating systems are stated to be a higher degree of complexity and the demand for electricity to run the pumps and controls. Fewer studies are focused on forced circulation systems specially d
16、esigned for cold regions. Wongsuwan and Kumar concluded that TRNSYS software predicts accurately the performance of forced circulation systems. Buckles and Klein used TRNSYS software to theoretically investigate different system configurations, and concluded that heat exchangers, with the same capac
17、ity, have similar performance whether placed inside or outside the storage. In 2009, Hobbi and Siddiqui used TRNSYS to theoretically optimize the design of a forced circulation hot water system for cold climates. As in other studies, the annual solar fraction was used as design parameter to be optim
18、ized . Of specific interest is the fact that there is a lack of studies investigating the retrofitting of existing water heaters in single-family houses for solar hot water application in cold regions. Retrofitting solar domestic hot water systems today commonly consists on series connecting the exi
19、sting system with a new solar storage upstream and using the existing hot water boiler as a backup heater . Some retrofitted systems make use of thermosyphoning and are therefore dependent on a new well performing storage for that purpose . Existing installations are often oversized in volume since
20、large design loads have been used before and because domestic hot water consumption has been decreasing . Combined with low levels of insulation, such over sizing causes large system heat losses. When using the existing storages as solar hot water storage the thermal losses in the system can be decr
21、eased since the solar water temperature is lower than the backup temperature. In cases where it is possible to know when the existing boiler is close to the end of its lifetime, it can be advantageous to change the existing storage. In this research, retrofitting of conventional domestic water heate
22、rs using forced circulation SWH systems for single-family houses in the Swedish climate was theoretically investigated. This was carried out by means of two pumps, one in the tank loop and the other in the solar collector loop with a heat exchanger in between. Four different system configurations were simulated in TRNSYS software and compared based on the annual solar fraction. Since forced circulation is used, almost
