What is Merit and how does it work??

Merit is a simulation program developed at the University of Strathclyde for calculating the potential of various forms of renewable technology (PV, DWT, solar collectors and solar concentrators) to meet the demand of single or multiple buildings. The demand profiles of various typical buildings are contained within a database and can be selected. These profiles may also be scaled up or down. New profiles may demand profiles may also be created. The demand may be from a single building or an entire village or town. Merit contains climatic data for various for a few locations around the world such as Glasgow, Milan and Flagstaff. Using the climatic data merit is able to create a supply profile for the various forms of renewable energy. For the renewable technology the amount of electrical generating units can be selected. Merit contains various technologies; for each technology set the critical parameters affecting the production of electrical energy are separate fields. Data can be imported from technical manuals or web pages such as the BP website for PV modules, thus new supply profiles are created. The demand side and the supply side are selected. Merit also allows the integration of auxiliary systems such as batteries and the grid. Merit then uses a matching tool which allows the selection of multiple supply and demand profiles as well as the auxiliary system and then matches these profiles. The output from the match is the excess and deficit in KWhr resultant from the selected combinations as well as the cost or electricity from the grid if the grid auxiliary is selected. Merit also produced a category of match ranging from 0 to 10 where 0 is no match and 10 is a perfect match.

In order to produce meaningful data, Merit calculates the difference between the supply of electricity, thermal and hot water against demand on an hourly basis. Merit matches supply and demand for each hour throughout the day. Thus the demand profile format is also on an hourly basis throughout the day. In stage eight of the methodology the electrical demand for the building was presented. It was necessary to convert the electrical demand into a format which is can be used by Merit, this means for several periods throughout the year the electrical demand calculated for the building during one particular day in a particular season is applied to many days. There are three main profile types in Merit; these are the weekday profile, the weekend profile and the holiday profile.

• There were two main options that would enable conversion of the electrical profile for the proposed building at Rottenrow input into merit. The first option was the creation of an entirely new profile; this would involve the input of a minimum of 24 values for the weekday, weekend and holiday periods which represent the demand for set periods throughout the year. This option was ruled out at the early stages as the basic electrical load calculations were on a daily basis. Thus converting daily electrical demands for use in merit would be time consuming. As Merit calculates on hourly time steps that the electrical demand is required to be as accurate as possible, if the daily electrical demand was simply converted to hourly demand by dividing the demand by the number of hours in the day the accuracy of the results would be open to question. It should be noted time constraints played a major factor in this project, if time constraints were not so prominent it would be possible to create an all new yearly profile. This would be the most accurate option. The calculated daily demand was considered relatively accurate thus the best option was to look for an electrical demand profile which had a daily consumption similar to the calculated value for the Rottenrow site daily consumption; the profile could then be scaled on an hourly basis. This would facilitate the creation of the necessary data within the merit demand profile throughout the year for the weekday, weekend and holiday periods and would be relatively accurate. The profile which already existed in merit that was closest to the profile created from the basic electrical calculations was the office profile. This was then scaled on an hourly basis towards attaining a daily demand which was close to that calculated from the basic electrical demand calculations. Detailing the exact values of the scaling and the time periods to which the daily profiles were applied would be of limited significance within this web page. The most important issue is the method used.
• The various hot water demand profiles which already existed in merit were analysed and found not to be close to the profile which was calculated based on the static heat load calculations. It was thus necessary to create a new profile based on those values from the static hot water calculations. Thus a set number of weekdays, weekend and holiday profiles were created based on the hourly consumption and applied set periods throughout the year in order to create an annual profile.

PV Analysis

The following bullet points detail the tasks involved in the determination if which PV cells should be include in the building.

• Select several different types of PV (I chose to select the PV’s based on those PV cells for which both cost and performance data could was available). The following web sites were used amongst others for data capture. www.windandsun.co.uk, www.bpsolar.com
• The values required in order for Merit to calculate the PV performance modules are; Open circuit voltage (Voc), Short circuit current (Isc), Maximum power point voltage (Vmp), Maximum power point current (Imp), Temperature at STC, Isolation at STC, Number of series connected, number of parallel, Panel width, Panel height and nominal peal power. These values are obtained from the above web sites and are then fed into Merit. This allows the calculation of the efficiency of the module, and enables Merit to produce a yearly electrical supply profile for the module.
• The remit for this project was the design of a minimum energy use building during the use phase towards the mitigation of carbon dioxide by reducing the consumption electricity derived from the grid.
• In core one it was stated that the most important category was the mitigation of CO₂, however in order to choose between various different PV cells cost has been used as a mechanism for achieving this.
• The cost of the PV system, not including inverter or battery costs is calculated using the annual payback method. This is defined as cost of the system*interest rate*(1+interest rate)^repayment period

((1 + interest rate)^repayment period) – 1)

An interest rate of 5% per annum was selected and a payback period of 25 years.

• The performance analysis at the Rottenrow site is based upon the Glasgow climate analysis conditions, however to provide an example of how the performance of PV various at different sites the Milan and Flagstaff (USA) climate data was used to provide a worthwhile example of how the viability of PV changes dependent on the site conditions. Although the viability is demonstrated by way of cost, the same environmental viability of the installation of PV at various different sites has also been used. The results which are of real significance within the spreadsheet below are those specifically for the Glasgow climate.

The flowing spreadsheet shows how the calculations were laid out.

PV PERFORMANCE WITH MERIT

§ The graphs above show how the cost of electricity obtained from the PV cells vary quite dramatically depending on the climate. More importantly, for this case study is can be seen that the PV modules which have the lowest cost/KWhr for the Glasgow analysis conditions is the BP585.

§ The Merit supply and demand matching tool was used for the BP585 PV module for various installed areas.

§ The orientation of the PV modules is set due south

§ The angle of the façade is set at 20 degree from the horizontal

The flowing spreadsheet details the calculations involved in determination of the optimum area which should be covered with the BP585 module type. The spreadsheet shows the steps involved in determining the optimum number of batteries for electrical storage.

PV DECISION MAKING PROCESS

The optimum area for the installation of PV modules was determined by calculating CO₂ saved by installing a certain area of PV when compared to the amount of CO₂ which would be released if building electrical demand was matched by using only electricity derived from the grid. In order to determine the CO₂ release from the supply of the electrical demand from the grid a figure of 0.43 kg CO₂ released per KWhr of grid electricity used was applied. This figure was obtained from the carbon trust. Refer to http://www.energy-efficiency.org/ The first graph, refer to graph 1 in the spreadsheet (the above link) shows how the reduction by percentage of CO₂ from grid values against the increase in area for the BP585 module. It can be interpreted that for a constant increase in the area covered by PV modules the amount of CO₂ saved begins reduces. It is estimated that the installation of more PV modules begins to have much less of an effect at around 600 modules; however the graph does not give an absolute critical value for the area of installed PV. In order to obtain a clearer indication of the most environmentally sound choice for the PV area graph 2 shows the way in which the percentage of CO₂ saved reduces with the installation of more PV cells. It has been stated in the life cycle section of this website that a life cycle assessment of the impacts of PV panels is a project within its self as is a determination of the emissions associated with the production of PV modules. It should be obvious that for every PV module which manufactured there is an associated release of CO₂. Thus as the effectiveness of installing higher areas of PV’s with respect to savings in CO₂ emissions when compared to using electricity from the grid begins to reduce substantially as the effectiveness of installing more PV cells on CO₂ emissions begins to drop considerably. Thus it is upon this philosophy that the choice to install 600 modules has been based.

The decision on how many batteries to use for the storage of electrical energy produced from the PV’s was less clear, in the final graph it can be seen that a reduction in savings of CO₂ with the use of more batteries begins to decrease at around 15 batteries. It was found that there is difficulty is defining exactly the amount of batteries which would be best suited towards contributing to the reduction in CO₂ emissions.

It should be stated that there are other balance of system components related to the installation of a PV system such as inverters. As these system components are necessary regardless of the amount of PV cells or PV area which is determined on the basis of optimum CO₂ emission savings there analysis is not considered in this web site. The determination of the size and type of inverter is arbitrary in terms of maximum CO₂ saving from the system.

Solar collectors

From the high-level selection tool it was found that the installation of solar collectors towards meeting a proportion of the hot water demand for the building could be an effective strategy toward combating the release of CO₂ from traditional fossil fuels. The fossil fuel use used for calculating the potential saving in CO₂ through the installation of solar collectors is natural gas; this was chosen as the most efficient heating systems tend to use natural gas as the fuel. As with electricity, a value is used that relates the release of CO₂ associated with burning 1kg of natural gas.

The primary question associated with the installation of solar collectors as with the installation of PV module was apparent; that was determining the optimal area of installed solar collectors. The technique used to find the optimal area of PV is used for solar collectors was to determine the amount of CO₂ which is saved for the installation of a certain area of solar collectors and to investigate how this varies by increasing the area of installed solar collectors.

The following spreadsheet gives an indication of the how the determination of the optimal area for solar collector installation was carried out as well as showing how the volume of storage associated with the solar collectors was achieved.

SOLAR COLLECTOR ANALYSIS, MATCHING HOT WATER DEMAND

The results in the above spreadsheet indicate that the optimum area of installed PV is 25.32 m2 with volume storage per collector of 0.15m2. The same principle of mitigation of CO₂ was applied to the decision making process for installed area of PV was applied to solar collectors. The decision making process was simpler for the solar collectors as the graphs shown above give a clear indication of the amount of CO₂ saved per annum.

Ducted Wind Turbines

Ducted wind turbines were not eliminated by the high-level selection tool and are thus further analysed. The orientation of the building and the surrounding topology will mean that installation of turbines is possible facing due south and due west only. The following spreadsheet details the calculations involving ducted wind turbines as well as the chosen area of BP585 PV modules.

DUCTED WIND TURBINES DECISION MAKING PROCESS

It is estimated that the maximum amount of ducted wind turbines facing both due south and due west should be around 14 on each side. This number was chosen based on the requirement to keep the turbines a set distance apart. If this distance is estimated to be around 2 metres, this equates to roughly 30 metres. This is around the space available for the installation of the turbines. It is actually not necessary to install the turbines a set distance apart however this would facilitate easier maintenance if required during the turbines operation.

The analysis method used for determining the amount of ducted wind turbines which would be best suited for optimum CO2 savings was that use for PV and solar collectors. It can be seen in the spreadsheet for the analysis of ducted wind turbines that there is a linear increase in the CO2 saving for a linear increase in the amount of ducted wind turbines, thus it is impossible to dictate the amount of turbines which should be installed by using this method.

It is estimated that the maximum amount of ducted wind turbines facing both due south and due west should be around 14 on each side. This number was chosen based on the requirement to keep the turbines a set distance apart. If this distance is estimated to be around 2 metres, this equates to roughly 30 metres. This is around the space available for the installation of the turbines. It is actually not necessary to install DWT turbines a set distance apart however this would facilitate easier maintenance if required during the turbines operation.

As yet there has been no further analysis of the technologies for supply of the space-heating requirement. The only technology which was not eliminated by the high-level renewable technology selection tool is that of Geothermal Heat Pumps (GHP). After some initial investigation of GHP it was decided that the further investigation of GHP is a difficult and time consuming undertaking. Issues such as analysis necessary for the underground pipe work for GHP is a project undertaking in its self. Thus due to time constraints surrounding this project the further study of integration of GHP into the building would need to carried out at a later stage.

Suggested renewable technology for the integration into the Rottenrow University building

• 600 * BP 585 monocrystalline PV modules with an area of 389 m2
• 15 * 415Ah 6V batteries
• 8 * MSC32 solar collector with area of 24.33 m²with primary tank storage of 0.15V³ preheat storage capacity and 0.05 V³ main storage tank.
• 14 DWT facing south and 14 DWT facing west.
• GHP system requires further investigation.

DETAILED ANALYSIS WITH ESP-r