Photovoltaic Water Heater: Introduction

A recent project I undertook on the property was to split our household water heater to be powered both by photovoltaic panels – that is panels that convert sunlight to electricity – as well as grid power. In this series of entries, I will go over some of the rational for the project, design considerations and specifics of my build.

Disclaimer:

This project involves water, electricity and temperatures at the water tank that are hot enough to scald if things are working well and really quite dangerous if you get things wrong.

You also need to consider things like your building codes, municiple codes and your insurance policy. At the very least, you will end up voiding the waranty on your water heater.

Overall, this is not a beginner project. If you aren’t already comfortable with plumbing and electrical work, this is not a great place to start. I’m not a plumber or electrician or engineer. I try to put forward the most useful information possible but please consider everything carefully and verify everything before trying anything like this at home. The only thing I can guarantee is that the potential hazards here are very real.

Rationale

South windows in the “great room.” … and it goes on like that.

The first projects on our property dealt primarily with shelter. The foundation of the house was repaired, and metal roofing put on. The house was gutted and the walls doulbed in thickness with staggered studs and lots of insulation. The South wall was re-framed and large windows put in to increase passive solar gains.

Our attention then moved to food production. First the raised beds and other garden beds went in. Then the chickens came along and that project really took off. This last year I built the greenhouse which has helped us produce even more vegetables than in the past. Alongside this, I hunt and between that and the chickens we haven’t bought meat for several years.

The further you go down any path, however, the more likley you are to hit the law of diminishing returns. Each incremental improvement starts to require more resources – be that time, money, energy or so forth, than the last improvement of a similar magnitude. We are starting to approach that point with food production, and while we will continue to develop our capabilities and the staging for the orchard is coming along, it really was time to look at other sections of our budget and life.

Energy was a natural one. On our property that means electricity, as the overhead involved in bringing gas to the property is more than it is worth for our modest needs. The house renovations did drop our electricity bills substantially and there is talk of adding a wood stove at some point, but in the meantime the heating of water represents a significant portion of our annual energy consumption.

We are not alone in this, as many households consume significant resources keeping large tanks of water heated. Tankless heaters are a good solution here, though they only seem to work properly with gas. With electrictiy, the draw of the appliance represents a significant portion of a household electrical service and there are mixed reviews as to how well the electric tankless heater – really tank heaters with volumes of only a couple gallons – can keep up with demands. So instead we all look at these big tanks of water constantly losing heat to the environment.

Photovoltaic Panels

Photovoltaic panels mounted on adjustable brackets to the roof.

One way to deal with this issue is to use solar power to heat the water. There are two ways to go about this – direct systems in which the water is directly heated by the sun and photovoltaic systems in which sunlight is converted into electrictiy and then this is used to heat the water.

Up until recently, the concensus on which approach to take has been all but universal. Directly heating water is vastly more efficient and the payback on even commercially produced and installed systems tended to be less than five years as opposed to the decades required to pay back the cost of photovoltaic systems. Better still, you could do a lot of DIY and build your own collectors and other components to cut costs even further.

Times they are a changin’, though. Photovoltaic panels have dropped substantially in price and there has been a boost in efficiency as well. Unfortunately, the traditional photovoltaic system with battery banks and inverters still suffers some of the same issues as they always have. Battery technology has come a long way but still represents a major source of system efficiency loss as well as a source of both ongoing expense and waste production as the batteries need replacing. Inverters, chargers and other circuitry also represent a source of loss, though these are now incredibly efficient and the loss is not particularly significant.

Photovoltaic Water Heater System

Schematic of the water heater system. The lower heater element runs off of solar power. The top element runs off household power.

If you are heating water with the electricity instead of storing it in batteries, though, the efficiency can be much better. Provided your final product is hot water and not an attempt to extract the heat as electricity through a heat pump or similar scheme, most of the wattage you produce through your panels is used to your advantage. In fact, the efficiencies on electric heating can approach 100% .. though there are some complexities here that we will get into later.

Lastly, there are some real advantages to photovoltaic systems over direct solar heating. You don’t have to worry about pumps, valves or other failable elements. You don’t have to worry about freeze or overheat protection. You don’t have to run plumbing through your building or worry about gravity, air bubbles or corrosion inside the system. Running electrical lines is generally much easier and the system takes up much less space inside the house.

In many cases, including our own, you don’t even have to add extra tanks or purchase tanks with heat converters inside. I just took my standard household water heater and split it to run on both solar and household power.

Water Heater Operation

Before we continue, a quick overview of conventional electric water heater operations. A standard tank will have two elements, one towards the top of the tank and one towards the bottom. Each is controlled by its own thermostat, which can be connected to each other and to the household power in a variety of schemes. The most common, and the one with which we started, is called non-simutaneous. In this scheme, the power first goes through an emergency cut-off that turns off the entire system if the water reaches a certain critical temperature. In this way, the system is rendered safe long before the water gets to the point where the temperature/pressure release valve installed on the tank is required to let off water or steam.

Non-simultaneous Water Heater Operation

1. Top element heats water to desired temperature. 2. Top element turns off, directs power to bottom element. 3. Bottom element heats the bottom of the tank to just below final desired temperature. 4. Once the temperatures have been achieved, the tank shuts off.

After passing through the cut-off, the power goes to the top thermostat(1). This thermostat is set to the desired final temperature of the water and maintains the top layer of the tank at this temperature. Hot water rises and so the hotest water is naturally kept at the top of the thermal gradiant inside the tank. After hot water is drawn out of the tank, the drop in water temperature is registered by the thermostat and the top heating element is turned on until the temperature has been raised to the set point, at which point that thermostat shuts off.

When the top element is on, no power is directed towards the lower element. Once that top thermostat shuts off, though(2), it directs power to the lower element(3). This element is traditionally set a few degrees lower than the top element and effectievely pre-heats the balance of the water in the tank so that the top element doesn’t need to work as hard to raise the water to its final temperature. If both the top thermostat and lower thermostat have met their set points, the system is off(4).

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