restart

After most of a year of delay, I have restarted my solar home project.

Yesterday (October 2), I gave a presentation on home solar power at a local parochial school, "Nuestra Señora de Carmen", in Hatillo, PR. I was very impressed by the abilities and interests of this group of 11^th graders. They asked a lot of interesting questions, and in this post, I will begin to address them. We start with anticipated power cost savings.

The simple calculation is to estimate how many kilowatt-hours per month my system will produce and how much electrical power will cost over the coming years. We are installing a 2kW system. We can reasonably expect to have full sun on the panels for 6 hours/day, 25 days per month, which adds up to 150 h/month. We currently pay 20¢ per kWh, so this will be worth $30/month. At this rate, we will reduce our electric bill by $360 a year, or $3600 over 10 years, and it will take about 20 years to pay off our ultimate construction cost, which is close to the 25-year 80% efficiency lifetime of the panels. (A total cost of $7200 includes the tax credit we expect to receive from the PR government.) In reality, it may take 25–30 years to pay off the system.

The above calculation makes a big assumption: that electricity costs will not go up. However, only a little over a year ago we were paying close to 30¢ per kWh, and this cost will increase in the coming years. The question is, by how much? This is a big problem to estimate, as electric power costs do not change with any predictable pattern. We can assume that these costs have bottomed out, given the state of the economy, the environment, and of known reserves of petroleum (see this link for estimates of future petroleum costs) (in PR there are currently no other sources of electricity). If electrical power averages 30¢ per kWh over the next 10 years, we will save $5400 rather than $3600.

I will do some more research on electricity costs. In future posts I will touch on changes in our plans, solar-thermal power, and wind power.

My Solar Home

Web resources:
English:
Puerto Rico Incentives for Renewables and Efficiency

Español
Ley 248 del 10 de agosto de 2008
Carta Circular Rentas Internas 08-13
Resumen de ley 248

Introduction
In August of this year (2008) the government of Puerto Rico announced tax incentives for the installation of solar energy systems in homes and businesses. These include both electricity and water heaters. They pay up to 75% of the cost through June 30, 2009, 50% during the following year, and 25% thereafter. Costs that exceed the taxpayer's tax liability can be carried over to following years, up to 10 years.

My goal is to install a grid-tied solar electricity system to provide for most of my house most of the time. I hope to reduce my electricity use by 1/2, and I hope to sell a little back to the utility (at 10¢/kW-h). Here in Puerto Rico we have adequate to good sun all year, so this should work well.

My home has a solar water heater, and I have been considering solar panels since we began building in 2002. This tax incentive has inspired me to move forward, though I have been studying the options for some time. An internet search identified a handful of solar energy retailers in Puerto Rico, and I send out several e-mail and webform inquiries. I received a response from PRGreenTech.com, and most of what follows came from that communication. I also have visited Casa Solar, PR in Bayamón.

Julio Correa, of PR Green Tech, visited my home on November 21. I was concerned by the fact that there is a tree-covered hill to the east and many afternoons are cloudy, both of which may limit sun exposure, but Correa did not feel these were such important concerns. Sun will be on the panels by between 8:30 and 9:00, even in mid-winter, when the shading is worst, and we should typically have 4-6 hours of full sun. The fact that Puerto Rico is 18N latitude also helps.

Correa works with 48 V DC systems, and the panels are 195 W, 24 V. Rather than connecting to the charging system in pairs, producing 48 V in full sun, he uses the panels in sets of 3, which will produce 48 V even in some degree of shade. The panels are polycrystalline, which, though they are less efficient, have two advantages: 1. they are more durable, and 2. (I understand that) their "turn-on" light level is lower than for monocrystalline (I have to confirm this).

My house has a few items that produce heavy loads. We will exclude the one air conditioner, which will continue to connect directly to the grid. It is a 18,000 BTU/h unit, which is rated for 1950 W. That excluded, we still have the following:
1. A water (deep well) pump. We always use water from a 600 Ga reservoir, which is supplied from the aqueduct authority. This is because water supplies here are quite unreliable (we recently had 2+ weeks with no water during the day, and only a trickle at night). This pump is rated for 1500 W.
2. A microwave oven. Correa says these are among the biggest problems for solar inverters.
3. A toaster oven.
4. Blowdrier
Happily, our oven and cloths drier are both connected to gas. The solar water heater has an electrical backup, but if we are using that, it means there hasn't been sun for a while, so we will be running off the grid in any case.

In future blog posts, I will describe the process by which we choose our solar energy system. These are divided up as follows:
1. Total power output of the panels.
2. Specifics of the charging system (voltage regulation)
3. Battery capacity (in A-h), costs etc. (batteries are not covered by the tax incentive law, so cost is critical)
4. Inverter capacity
5. Grid-tie system
6. Questions:
- At what point in the battery depletion do we switch off the solar powered system and go to grid?
- How much solar panel capacity is needed to provide sufficient charge to the batteries to run until charging re-commences?
- How much energy consumption (A-h) will we need, and how to we convert that throughout the system?
- How do we make sure the system is expandable, so we can add capacity later for running the air conditioner?