The hot water tank is a 300 litre stainless steel tank. Stainless steel is better at reducing stratification (minimising mixing) because it conducts less heat compared to copper. Different grades of stainless steel exist for different types of water (hard/soft). One also needs to check the type of welds used on the tank as some can not cope with certain water types.
The tank was modified to allow me to connect the solar PV water heating system (previous blog) in the future using thermosyphonic action i.e. hot water is lighter than cold water so it naturally flows from the top of the tank to the bottom (Reducing the need for pumps). The DC power from the solar panels will be connected to electric heating elements. As the solar power varies the heating elements will adjust the output power through a control unit I am developing.
IfI install a solar hot water system or another method in the future this will be done with a plate heat exchanger rather than a coil. I installed extra connections on the tank for this reason. The reason for the heat exchanger is that the tank will heat from the top down. A plate heat exchanger looks like the following
If one opts for a coil it creates turbulence while heating the tank. I found it difficult to find a tank manufacturer who will install the correct surface area of a coil for a climate like Ireland. I feel most are designed for hot countries like Spain where the sun shines and stays shining. If one opts for a coil rather than a plate heat exchanger one requires a large coil surface area to ensure that most of the solar energy transferred in the least amount of time and the temperature returned to the hot water solar panel is at a minimum. In this way the hot water solar panel can operate at its maximum efficiency.
I really like the possibility of converting light to electricity. In the spirit of innovation I will simplify the PV system and reduce the cost by returning to using DC power directly from the PV panels. As I have no intention of transmitting power around the countryside this is another reason for staying with DC.
The passive house standard I feel helps in this approach by reducing the energy required in a house to a very low level which creates the synergy to make this leap for me to design a DC power heating system for our house.
Solar Background Information
The energy from the sun varies throughout the year due to cloud and the amount of air mass (AM) it has to pass through to reach the earth. When the angle of the sun is low in the sky the solar energy has to pass through more air which means less solar energy is available to convert light into electricity (example winter months).
When a PV manufacturer quotes the output power of their PV panel the international standard is to quote the output power at an air mass of 1.5 . An air mass of 0 is know as outer space where no air exists (AM0) . This is where the maximum energy can be captured if one lived in space. An air mass of 1 is when the sun is at its highest point in the sky (AM1) when on earth. The AM value of 1.5 is around 48 degrees off the highest point on earth.
Measuring the Solar Energy (W/m2)
In the winter months from tests I have carried out with my pyranometer ( a device that measures the solar energy-see below) the solar power I am recording with a data logger was between 0 and 400 watts per m2 (It can be higher on sunny days). In the spring/summer months the power can reach 1000 watts per m2 and more.
Harvesting Solar energy in the winter months for me is the priority which will entail the correct location and angle of the solar PV panel for the winter sun. The strategy is to try capture as much of the winter sun as possible by balancing the solar gain of the glass in the south windows of the house (part of the passive house performance phpp calculations) and supplement this with the DC (Direct Current) electricity from solar PV panels to provide space and primary heating etc. It is very noticeable at this stage of the build the real benefit of gathering energy from the winter sun through the glass. (See previous blog on performance data 23/05/2015).
In order to give an idea of the solar energy available I recorded the irradiance when the sun was behind a dark cloud (see image below). This equates to around 200 watts per m2 solar energy. When the sun came out from behind the cloud it reached over 1000 watts per m2 in the month of April.
Sun behind dark grey cloud is approximately 200 W/m2 of solar energy (above image). Most inverters start to loose their effeciencys at this point.
Sun behind dark grey cloud is approximately 112 W/m2 of solar energy on the typical overcast day. Most inverters would stop working at this level.
Below is an example of the changes that take place on a sunny/cloudy day in May
In the above chart one can see an example of how difficult it is for an inverter to keep working efficiently (they work efficiently from approximately 200 w/m2). The bottom line on the left is 100 watts/m2, The top line is 700 watts/m2 (click on the image to see more detail) . In the winter time values from around 50w/m2 to 200w/m2 are the lower limits and the upper limits are around 600w/m2.
Self Build Homework (Develop a DC powered Solar Harvesting Unit)
For the above I do need to find a way of maximizing the output power of the PV panels as the iradiance varies. For this I need to develop a simple black box (a small amount of simple components) that will match the solar energy created by the PV panels and maximise the output over the winter months. I am close to having a working prototype to see this in action (all tests look good so far ).
The equipment to be purchased for the above will be 4 solar panels and the mounting brackets. 4 solar panels will provide around 1 KW of power (max). This will cost around €1000. More groups of these will be added in the future. (If any one has 4 spare panels to loan so that I can test the control unit I am building – please let me know.)
In essence I plan to create what I call a DC Solar Harvesting Unit (DCSHU) that will have specific electric power functions around the house.
The pitch of both roofs is 12.5 degrees. The architect choose this in order to optimise the solar gain during the winter months and ensure both buildings receive adequate light.
There is a roof overhang of 1.8 metres in order to control the solar gain during the summer. (See below)
The build up of the roof from the inside is : Plaster Board, 40-100mm service cavity, airtight layer, 400mm metac isover semi rigid 0.34 k insulation, bitumen impregnated wood fibre board, membrane, ventilated cavity, OSB, DELTA®-TRELA membrane and then zinc.
We thermally broke the rafters where possible see above.
Planning Permission Factors
In our planning permission reference was made to standing seam. This restricted the options for controlling costs later on. (A lesson for others). As it turned out for our design there were no variations in the roof (ie no openings, different pitches, etc and this kept the costs under control. )
If one is designing from scratch note that tiles/slate options start with a minimum pitch of approximately 12.5 degrees (example: the melodie single pantile). During the design stage if one can simplify the design of the roof by minimising openings for roof lights, ventilation flues and any architectural details this will keep the costs under control.
An early idea I explored was to use amorphous solar electric panels (PV) built into the roof -I was unable to come up with a solution in the time frame and deal with the potential risks such as Fire/Insurance/Waterproofing and Hygrothermal issues of an intergrated roof solution. I will revisit this idea in the future.
When one selects low pitch roofs the options I am aware of are a green roof, EPDM (ethylene propylene diene terpolymer), and metal roofs. I considered the green roof but the hydrothermal analysis using Wufi software necessitated a different build up of the roof layers and the other reason was the need to apply for a change in our granted planning application.
Zinc can be placed on spaced untreated wooden battens without a membrane – a cost saving is possible using this technique.
All fixings nails and screws are either grade 2 or 4 stainless steel.
New Roof Products
In the last month I came across a promising roof and wall facade system that acts as a solar hot water panel and it uses the drainback system (see previous blog on drainback system). The company is called http://www.aventa.no . As previously discussed it is too late for me to plan for this . What needs to be clarified is the cost of storage and panels .
When one is getting a quote from any zinc installer ensure you specify the same product. I found that the Zintek(be careful there is another name that sounds similar called Zintec but it is not zinc) was cheaper than Rheinzinc and the installers usually know which zinc is good. Ask for their opinion (as they are working with the material). Think whether you can use non patinated (natural) zinc in some places as it is roughly a €1 cheaper per kg. The non patinated zinc eventually returns I believe to the same colour as the patinated zinc. We were advised not to use non patinated zinc in areas where different weathering could occur. So for all the fascias/sofit and edging details visible we used patinated zinc. As zinc is a traded commodity on the stock market prices vary.
The zinc detail for the gutter were as follows. A ventilation/insect grille can be seen below.
The ventilation/Insect grille on the overhang is as shown below. A detail to satisfy the engineer and the zinc installer was agreed as proper ventilation and air flow was required for the roof.
Roof Overhang Zinc Detail Passive House
I must do more research on this as little appears to be available..
There are two basic types of solar hot water systems that collect heat from the sun to heat water. One is called a Drainback System and the other is called a Pressurised System.
I have decided to use the drainback system because :
it has the least amount of components
it is easy to maintain,
it does not suffer from stagnation issues
it has a longer life cycle
it has a higher efficiency
it is simplistic in design
The disadvantage is that it is not typical in Ireland and it requires correct installation.
The Drainback System
There are commercial and domestic drainback systems. The system I intend to use is a domestic unvented drainback system.
The choice of solar panel is important-the drainback system requires a flat-plate solar collector panel using risers (vertical pipes in the layout of a harp design-see sketch below) of around 12 mm (this might be a challenge to source).
Water (pure or non-hardened) with no glycol is used. Water is a better conductor of heat thus the efficiency is better (we need all the help we can get in the Irish climate).
Vacuum tubes are typically not available for use with the drainback system. My view on vacuum tubes is that they are equivalent to light bulbs on your roof (they use the vacuum principle like light bulbs which means they can fail). So from a cost point of view and maintenance point of view they are not being considered.